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CEL and OBD II Code P1123


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I have a 1999 996C2 Coupe with Manual Transmission that recently registered a Check Engine Light. After exhaustive (and quite fruitful) searches on this forum, I purchased an OBD II analyzer thanx to the excellent advice of various folks here on Renntech (free plug - I bought the DigoMoto unit which is both inexpensive and very easy to set up and use), and was able to retrieve the fault code, which was P1123 - Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Lean Threshold. Loren has given out much info on this problem on previous posts, and he says that this problem is symptomatic of one of three things: 1. fuel pressure too high; 2. fuels injector(s) leaking; 3. EVAP canister purge valve open.

I cleared the CEL from the analyzer just to see how quickly it would reappear. It was 2 weeks of driving before it reappeared, and performance does not appear to be adversely affected. The CEL finally came on again, with the same fault code, so I am left wondering about the debug sequence for this problem. Other folks seem to have suggested that the MAF can also give this code, but since I only see the code for one bank of cylinders, I suspect this is not the case. Does anyone have any suggestions for how to attack this? Call me cheap, but I am not looking forward to a trip to the dealer for something like this, especially when the people on this forum seem more knowledgeable sometimes than my local dealer. Any help/advice would be appreciated.

Edited by mumeh
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What is the voltage reading on the scanner for the MAF? Or the g/sec reading depending on what it reports? Also, look at the voltage graph for the O2 sensors (Primary) to see if they are quickly switching between approximately .05 volts and .85 volts after the engine is warm and idleing at about 2,000 rpm. A slow voltage change on Bank 1's o2 sensor will identify it as the culprit.

Normally when you get a P1123 you also get a P1125 which is the error for Bank 2. You, however are just showing that bank 1 is running rich and if this is true then it is unlikely that the MAF is at fault because that will effect BOTH banks. You could, however be right at the maximum LEAN thresshold on both O2 sensors, but bank 1's is a little more worn than is bank 2's and therefore you have not gotton the error P1125 yet but will eventually.

Knowing what the MAF reading is will be a great help in more diagnosis. Need reading after engine is warmed up and at idle. I also need the exact RPM at idle.

Edited by 1999Porsche911
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  • 2 weeks later...

After resetting the CEL, I finally (again) got a CEL signal, and it is P1123 again. I did go in and look at the characteristic of the O2 sensor, and it DOES indeed vary rapidly between the two threshold voltages you mentioned. I still have not observed a P1125 error, and the RPM is at 700 steady. I have attached my MAF readings obtained via the Digimoto unit used to monitor OBD II. Any suggestions for how I might go about narrowing the problem? I DID try dipping the MAF in isopropyl alcohol and replacing, to no avail.......

post-3935-1106373464_thumb.jpg

Edited by mumeh
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If the MAF and O2 sensor has been eliminated as the cause, then you may a have an injector that is leaking on bank 1.  This can also cause a P1123.

What are your fuel trim settings for each bank?

Some scanners will show what the engine parameters were when the CEL was set.  Take a look at that, something may jump out at you.

I was unable to get a sensor snapshot of the engine readings when the error code occurred, but I have been able to verify that it is only the P1123 that is tripping the CEL. I have attached the fuel trim settings per your instructions. They show about 30 seconds of idle, peppered by one or two engine revs just to see the difference. Thanx again for your help. I am approaching a smog test in CA, so you can imagine why I want this resolved particularly quickly. Any help is appreciated.

post-3935-1106521256_thumb.jpg

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Your MAF looks to be fine at idle an fuel trim on bank1 doesn't look out of wack. However, how does this fuel trim compare to that of bank 2?

Since you are getting only the 1123, I would guess it either has to be a leaky fuel injector (which I think is unlikely) or may just a bad O2 sensor. You might want to simply buy a new O2 sensor (Bank 1 before the CAT), or switch bank 1 with bank 2 to see if the error code follows the sensor. The sensor only costs about $120.00 and if you don't need it now, you will someday. Replacement if very simple using a open end wrench and should take you no more than 20 minutes. This is probably what Porsche would do, but also charge you $135.00 an hour.

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Your MAF looks to be fine at idle an fuel trim on bank1 doesn't look out of wack.  However, how does this fuel trim compare to that of bank 2?

Since you are getting only the 1123, I would guess it either has to be a leaky fuel injector (which I think is unlikely) or may just a bad O2 sensor.  You might want to simply buy a new O2 sensor (Bank 1 before the CAT), or switch bank 1 with bank 2 to see if the error code follows the sensor.  The sensor only costs about $120.00 and if you don't need it now, you will someday.  Replacement if very simple using a open end wrench and should take you no more than 20 minutes.  This is probably what Porsche would do, but also charge you $135.00 an hour.

Good advice, and thanx for the help. I have the sensor on order as we speak. I will post the results here once I get it in hand.

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  • 4 weeks later...
Your MAF looks to be fine at idle an fuel trim on bank1 doesn't look out of wack.  However, how does this fuel trim compare to that of bank 2?

Since you are getting only the 1123, I would guess it either has to be a leaky fuel injector (which I think is unlikely) or may just a bad O2 sensor.  You might want to simply buy a new O2 sensor (Bank 1 before the CAT), or switch bank 1 with bank 2 to see if the error code follows the sensor.  The sensor only costs about $120.00 and if you don't need it now, you will someday.  Replacement if very simple using a open end wrench and should take you no more than 20 minutes.  This is probably what Porsche would do, but also charge you $135.00 an hour.

I replaced the O2 sensor, and after some hours of operation, the CEL came back on, with the P1123 error again. Should I be looking at leaky injectors now? Any advice helpful.

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  • 4 weeks later...

Hi;

My boxster 1998. Aprox 60K miles.

CEL came on with P1123 code, Mechanic change O2 sensor. CEL came back after driving couple weeks. Showing now codes P1125 and P1123. Similar to problem listed on this posting. Sounds like he has much more knowledge about Porsche than I.

Will keep watching posting to see if he resolve his problem.

Thanks in advance for any more information and help you can give us.

Golfhom

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Hi;

My boxster 1998.  Aprox 60K miles. 

CEL came on  with P1123 code, Mechanic change O2 sensor.  CEL came back after driving couple weeks.  Showing now codes P1125 and P1123.  Similar to problem listed on this posting.  Sounds like he has much more knowledge about Porsche than I. 

Will keep watching posting to see if he resolve his problem. 

Thanks in advance for any more information and help you can give us.

Golfhom

Funny this. I replaced the O2 sensor, reset the error code AGAIN and for a while the problem appeared to be gone, but later the CEL recurred and now I get both P1123 and P1125. My scan tool is able to detect multiple codes. My hat is off to Porsche for consistency between the 996 and Boxster emission systems, but other than that I am irritated and perplexed. Where do I go from here? My idle is quite steady and engine performance appears to be very strong. I just have that darn CEL light on quite regularly. As mentioned before, I am loathe to approach the dealer unless I think that they can solve the problem any faster.

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I have still got the P1123 & P1125 error codes on mine, but NO CEL. Checked fuel pressure - perfect. I would check the EVAP canister purge valve if I knew where the heck to find it. I have just tried a bottle of injector cleaner in the fuel tank, in the vague hope that this will cure it. I'll let you know if it works for me. Meantime, anyone know where the EVAP canister purge valve is located (inlet manifold perhaps), and what it looks like?

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First thing I see here is that the sensed airflow is EXTREMELY low. It looks as if your air mass sensor is only reading 5 grams per second; around 13-14 grams per second (at idle, no consumers) is pretty normal. Although when the air mass sensor usually fails it will read higher air flow than reality, they do also fail in the opposite direction (or seeing less airflow as well). The probable reason why you didnt have P1125 before is that you werent at the threshold yet, but were very close and the last bit of driving put you over the limit and tripped the P1125.

While I am familiar with what STFT and LTFT mean (which are are generic OBD2 terms), I dont 100% know how to relate "Long term and Short term fuel trim" to the actual measurements Porsche uses for this car which are TRA (idle range) and FRA (off idle range) but I would imagine that if you disconected the battery (which will reset your idle range adaption values) and let the car idle for 10 to 15 minutes with your tester on, you would see the STFT numbers go positive pretty fast, though I dont know for sure as again, I'm not sure how those generic terms relate to Porsche's terms.

I would lean towards your air mass sensor having failed. If I didnt see such low air mass readings here, I would recommend you look in another direction but 5 grams per second is way too low. If you look at your LTFT its pretty much maxed out. Im thinking the reason for this is your air mass sensor is seeing WAY less air than is really going into your engine. Therefore its only adding enough fuel for that small amount of air. Your oxygen sensors are seeing this and saying "HEY DME, add more fuel!" As you know, the DME can only add so much fuel before the check engine light is illuminated. Pretty much the only reason you need to repace oxygen sensors is when you have heater circuit faults and ageing faults.

Hope this helped.

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First thing I see here is that the sensed airflow is EXTREMELY low.  It looks as if your air mass sensor is only reading 5 grams per second; around 13-14 grams per second (at idle, no consumers) is pretty normal.  Although when the air mass sensor usually fails it will read higher air flow than reality, they do also fail in the opposite direction (or seeing less airflow as well).  The probable reason why you didnt have P1125 before is that you werent at the threshold yet, but were very close and the last bit of driving put you over the limit and tripped the P1125.

While I am familiar with what STFT and  LTFT mean (which are are generic OBD2 terms), I dont 100% know how to relate "Long term and Short term fuel trim" to the actual measurements Porsche uses for this car which are TRA (idle range) and FRA (off idle range) but I would imagine that if you disconected the battery (which will reset your idle range adaption values) and let the car idle for 10 to 15 minutes with your tester on, you would see the STFT numbers go positive pretty fast, though I dont know for sure as again, I'm not sure how those generic terms relate to Porsche's terms.

I would lean towards your air mass sensor having failed.  If I didnt see such low air mass readings here, I would recommend you look in another direction but 5 grams per second is way too low.  If you look at your LTFT its pretty much maxed out.  Im thinking the reason for this is your air mass sensor is seeing WAY less air than is really going into your engine.  Therefore its only adding enough fuel for that small amount of air.  Your oxygen sensors are seeing this and saying "HEY DME, add more fuel!"  As you know, the DME can only add so much fuel before the check engine light is illuminated.  Pretty much the only reason you need to repace oxygen sensors is when you have heater circuit faults and ageing faults. 

Hope this helped.

PTEC:

His MAF reading is fine at 5.34 or so. If he is fully warmed up and his idle is 700 it is common to see such a low reading. A reading of 13-14 grams/s at idle would definitely set an error code. You may see a reading of 11 or slightly more when the car is just started and cold. Also, the fuel trim readings (he only shows bank 1) look fine. I would like to see what the other bank reads.

His LTFT IS NOT even near maxed out. (25% is limit and he is showing only 9%. I run -11 and -18 on my engine). The error code is NOT telling him the engine is running lean, but is running rich, and the O2 sensor is telling the DME to shut down fuel delivery. If the MAF was reading too low, the DME would be told to reduce the amount of fuel injected and the engine would be running too lean and the o2 sensors would have to tell the DME to increase fuel. This would give the error codes 1124 aqnd 1126 is the sensors tried to adjust past their range limit. I would like to see the readings of the primary o2 sensors at idle.

He may have a bad EVAP valve, a leaky injector or too high of fuel pressure. It may also be a faulty ground as the operating parameters he has shown us look fine.

Edited by 1999Porsche911
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PTEC:

His MAF reading is fine at 5.34 or so.  If he is fully warmed up and his idle is 700 it is common to see such a low reading.  A reading of 13-14 grams/s at idle would definitely set an error code.  You may see a reading of 11 or slightly more when the car is just started and cold.  Also, the fuel trim readings (he only shows bank 1) look fine. I would like to see what the other bank reads.

In my experiance (virtually zero with generic scan tools), I have never seen a air mass reading that low, ever. Perhaps again this is just something that generic scan tools display differently than the PST2/PIWIS. Either way, the car is getting MORE air than it thinks it is as the fuel trim is in the plus.

His LTFT IS NOT even near maxed out.  (25% is limit and he is showing only 9%.  I run -11 and -18 on my engine).

In that case, here is my question to you. If you state that 25% is the threshold for LTFT, then why does he have a check engine light and his LTFT is at 9%?

The error code is NOT telling him the engine is running lean, but is running rich, and the O2 sensor is telling the DME to shut down fuel delivery.

I disagree. The car is having to add fuel. Look at his LTFT number, its positive. The o2 sensor is reporting a lean a/f mixture and the DME is trying to adapt by adding fuel, hence the positive LTFT number.

If the MAF was reading too low,  the DME would be told to reduce the amount of fuel injected and the engine would be running too lean and the o2 sensors would have to tell the DME to increase fuel. This would give the error codes 1124 aqnd 1126 is the sensors tried to adjust past their range limit.  I would like to see the readings of the primary o2 sensors at idle.

Exactly. If the air mass sensor only reports 5 gm/s of air flowing into the engine, its only going to inject enough fuel to burn 5 gms of air. If the air mass sensor is defective and reading low, there is going to more air going into the engine than it thinks. The mixture will be lean, the o2 sensor will report this, the DME will add fuel up to its threshold, and the CEL comes on. P1123 and P1125 are the LEAN threshold, the car is too LEAN for the DME to adapt.

He may have a bad EVAP valve, a leaky injector or too high of fuel pressure.  It may also be a faulty ground as the operating parameters he has shown us look fine.

If he had any of those problems his fuel trim numbers would be negative, as the DME would be subtracting fuel to compensate for the extra fuel that a bad evap valve, leaking fuel injector, or too high of fuel pressure would deliver. The fact this his fuel trim numbers are positive screams either bad maf or vacuum leak.

Thats my two cents.

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PTEC:

His MAF reading is fine at 5.34 or so.   If he is fully warmed up and his idle is 700 it is common to see such a low reading.  A reading of 13-14 grams/s at idle would definitely set an error code.  You may see a reading of 11 or slightly more when the car is just started and cold.   Also, the fuel trim readings (he only shows bank 1) look fine. I would like to see what the other bank reads.

In my experiance (virtually zero with generic scan tools), I have never seen a air mass reading that low, ever. Perhaps again this is just something that generic scan tools display differently than the PST2/PIWIS. Either way, the car is getting MORE air than it thinks it is as the fuel trim is in the plus.

His LTFT IS NOT even near maxed out.  (25% is limit and he is showing only 9%.  I run -11 and -18 on my engine).

In that case, here is my question to you. If you state that 25% is the threshold for LTFT, then why does he have a check engine light and his LTFT is at 9%?

The error code is NOT telling him the engine is running lean, but is running rich, and the O2 sensor is telling the DME to shut down fuel delivery.

I disagree. The car is having to add fuel. Look at his LTFT number, its positive. The o2 sensor is reporting a lean a/f mixture and the DME is trying to adapt by adding fuel, hence the positive LTFT number.

If the MAF was reading too low,  the DME would be told to reduce the amount of fuel injected and the engine would be running too lean and the o2 sensors would have to tell the DME to increase fuel. This would give the error codes 1124 aqnd 1126 is the sensors tried to adjust past their range limit.  I would like to see the readings of the primary o2 sensors at idle.

Exactly. If the air mass sensor only reports 5 gm/s of air flowing into the engine, its only going to inject enough fuel to burn 5 gms of air. If the air mass sensor is defective and reading low, there is going to more air going into the engine than it thinks. The mixture will be lean, the o2 sensor will report this, the DME will add fuel up to its threshold, and the CEL comes on. P1123 and P1125 are the LEAN threshold, the car is too LEAN for the DME to adapt.

He may have a bad EVAP valve, a leaky injector or too high of fuel pressure.  It may also be a faulty ground as the operating parameters he has shown us look fine.

If he had any of those problems his fuel trim numbers would be negative, as the DME would be subtracting fuel to compensate for the extra fuel that a bad evap valve, leaking fuel injector, or too high of fuel pressure would deliver. The fact this his fuel trim numbers are positive screams either bad maf or vacuum leak.

Thats my two cents.

PTEC:

The "Lean Threshold" code means that the O2 sensors have reached their limit to instruct the DME to LEAN out the fuel mixture. It DOES NOT mean that the fuel mixture is lean. You are loking at these error completely backwards. The DME is being told that the engine is much too rich and the sensors cannot remove enough fuel to get it to the proper level.

Each and every arguement you are making is the complete opposite of what is happening. The arguments have some truth to them but only for a LEAN running engine which is not the case here. This engine is running rich.

The MAF is reporting 5.34 g/sec of air and assume that the amount of air is really 7 g/sec. The DME will inject fuel based on the 5.34 number. The O2 sensors will determine that there is actually more air in the mixture and therefore a reduction in voltage will occur (rich threshold) and the DME will increase the amount of fuel. If the sensor cannot reduce voltage enough to get the mixture correct, a RICH Threshold" code 1124 0r 1126 will occur. This tells you that the engine is running LEAN.

It doesn't matter what scanning tool you use if the both report g/sec of airflow. The fuel trim varies in all cars and does not necessarily mean that there is a problem. In my car, I am running much bigger injectors than stock and therefore, my LTFT will always show a large negative trim percentage when idling than does a stock engine. The LTFT simply reports how much the injector pulse is being reduced or increased on average.

THE LEAN and RICH threshold terms can be confusing. In the case of O2 sensors it means that the sensor is tring to reach the threshold and not reporting on it. When an O2 sensor goes towards the rich threshold (low voltage) it is increasing fuel and when it goes towards lean threshold ( higher voltage) it is removing fuel.

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FWIW - The DME diagnostic section of the manual shows the Mass Air Flow should be 17kg/hr +/- 2.5

14500/3600=4.02 g/sec

17000/3600=4.72 g/sec

19500/3600=5.41 g/sec

5.34 is definitely not low, and appears to be within tolerance.

Loren, thanks for the drawing of the EVAP canister purge valve - you had sent it to me before. I assume it is attached to the inlet manifold, but before I start delving into it, does anyone know what it looks like, and is it serviceable? A photo would be wonderful. Tool Pants seems to have a photo of every component on his Boxster - how about this one?

Edited by Richard Hamilton
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Answer me just one question, please. If he has any of the problems you guys mentioned (fuel pressure too high, purge valve stuck open or leaking injectors), then why are his fuel trim numbers positive?

Edited by PTEC
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Answer me just one question, please.  If he has any of the problems you guys mentioned (fuel pressure too high, purge valve stuck open or leaking injectors), then why are his fuel trim numbers positive?

The fuel trim reacts to many things. First off, no 2 injectors have the same flow. If the average flow of the 3 injectors on one bank in slightly higher than the DME is programmed for, then the LTFT will show a negative number on a normally running engine. If the average flow is less than expected, the LTFT will show a positive number. This is all in an attempt to get the fuel injection to what the DME wants.

Basic Operation:

The engine is started and the DME injects a specfic amount of fuel into the cylinders based on what it learned and stored in the LTFT tables the last time the engine was run. When the engine warms up and goes into CLOSED LOOP, This is what happens:

The MAF tells the computer how much air is being ingested and the DME tells the injectors to inject the amount of fuel necessary to bring the air/fuel ratio to 14.5:1. THIS CALCULATION TAKES INTO ACCOUNT THE SETTINGS OF THE LTFT. Next, the air/fuel ratio is measured from the exhaust by the O2 sensor which will make an adjustment to the STFT to either increase or decrease the amount of fuel injected. This continues and after awhile, the STFT adjustment is transfered to the LTFT tables and the STFT returns to ZERO until further adjustment is necessary.

ie. If your engine is running lean, the O2 sensor will increase the STFT by a specific amount; let's say 10%. If this increase continues for awhile, the 10% increase will transfer to the LTFT and the STFT will be ZERO. This is necessary, because a car that is started cold looks at the LTFT for what amount of fuel to use above what the DME is programmed for. In this example, if this table was not set, you would be running lean until the car warmed up and the O2 sensors could control fuel again.

My car has a drastically different LTFT setting for each bank. Because I am using large injectors and therefor the flow is much more than stock, my LTFT currently read: Bank 1 -18% and Bank 2 -9.8 %. This simply means that bank 1 injectors flow more than do bank 2 and that the flow of moth banks is more than necessary for normal vehicle operation. Both are well within the -25%/+25 threshold.

The P1123 and P1125 errors really have nothing to do with the fuel trim, but only with the voltage readings of the O2 sensors. The allowable voltage of the sensors is between .01 and .9 volts. If the sensor goes outside this range, a code is generated. The voltage goes lower when the fuel/air mixture is lean and goes high when it is rich. This voltage change directly effects and changes the STFT only. Higher voltage will decrease the STFT and lower will reduce it. Apparently the sensor went beyond their upper voltage limit and caused the codes.

You have to remember that many of the computer generated codes do not give you the underlying cause for the error, but only a place to start looking. You CAN conclude tho, that these codes are accurate in the reporting of the operation of the O2 sensors. Therefore, because they exceeded their LEAN threshold (high voltage limit) Either the sensors are bad, or they are reading a rich mixture accurately but cannot adjust enough. So you eliminate bad sensors and then simply go through the old fashion steps of determining the many things that can cause an engine to run rich and eliminating them as the cause, one by one.

Sorry for the long reply...it must be the laxative I took earlier. Hope it make sense.

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Before I reply to your long post above I need to ask you another question (socratic method, I guess).

If you state that the adaption threshold is 25%, why does he have this fault?

Oh and by the way, P1123 and P1125 are the exact same fault but are for the rich threshold.

The P1123 and P1125 errors really have nothing to do with the fuel trim...

Call your dealeship and ask to talk to a technician. See what they say when you tell them that.

Edited by PTEC
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Before I reply to your long post above I need to ask you another question (socratic method, I guess).

If you state that the adaption threshold is 25%, why does he have this fault?

Oh and by the way, P1123 and P1125 are the exact same fault but are for the rich threshold.

The P1123 and P1125 errors really have nothing to do with the fuel trim...

Call your dealeship and ask to talk to a technician. See what they say when you tell them that.

I really don't know what to tell you. If you are convinced that P1123 and P1125 are set because the engine us running lean, then you have been given incorrect information by someone or you hade a mistake in your typing. P1123 is for Bank 1 and the other for Bank 2. These error codes are because the sensors have reached beyond their LEAN threshold limit and the errors indicate the engine is running rich.

This is not opinion, but fact. You would be wasting your time looking to solve a lean condition in your engine because you have evrything backwards. There are several books available that you can read that will explain the function of the O2 sensors, Reading them might help you understand it better. I am also sure that you could find some sites on the web that will validate what I have told you.

If you got your information from a Porsche Technician, he might be playing you; or like so many of them, know little about the sensor operations on these cars.

Edited by 1999Porsche911
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Well, as the guy who started this thread, I would be remiss in my duties if I did not thank everyone for this wealth of information, as I am learning a TON about the closed loop operation of these engines. Everyone has taken great pains to make their points in a way that the common man can understand, and these discussions actually make sense to me. While I remain frustrated about my inability to solve this problem, I AM getting a lot of information and know more about this car than I did. Putting aside the lean vs. rich argument for a moment, can anyone chime in the troubleshooting sequence I have put together?

1. Disconnect battery for extended period of time to reset internal engine parameters. This is cheap and easy to do, and should do no harm.

2. Check evap cannister circuit and make sure that everything seals nicely. Again, cheap and easy.

3. Replace MAF Sensor. The fact that first Bank 1 and then Bank 2 showed error codes makes me suspicious that leaky injectors are not the most likely cause. For the longest time, I just received Bank 1 errors, and only recently did I start to see Bank 2.

4. Look into injectors. Ouch!

Suggestions?

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