CEL and OBD II Code P1123

[1999Porsche911]

mumeh:

Suggest you reset the computer and drive the car for a couple of cycles. Then, hoping the CEL has not come on yet, make a log of all the sensor readings, particularly the MAF, 4 O2 sensors, rpm's STFT and LTFT engine temp and timing....all at idle on a fully warmed engine. Then, if the CEL comes on again, make another log of the sensor readings. Your software may also give you a record of what the major sensors were doing whenthe CEL comes on. If so, report that to us also.

You engine setup is completely stock, I presume?

I am also curious what the MAF reading is right after startup on a cold engine.

Edited March 21, 2005 by 1999Porsche911

[mumeh]

mumeh:

Suggest you reset the computer and drive the car for a couple of cycles.  Then, hoping the CEL has not come on yet, make a log of all the sensor readings, particularly the MAF, 4 O2 sensors, rpm's STFT and LTFT engine temp and timing....all at idle on a fully warmed engine.  Then, if the CEL comes on again, make another log of the sensor readings.  Your software may also give you a record of what the major sensors were doing when the CEL comes on.  If so, report that to us also.

You engine setup is completely stock, I presume?

I am also curious what the MAF reading is right after startup on a cold engine.

<{POST_SNAPBACK}>

Thanks for the help. I will gather this data and report back. It may take me a week or so, as I don't drive the car much during the week. Unfortunately my software does NOT let me snapshot sensor data during the CEL trigger, so we'll just have to rely on the static information. And yes, the car is bone stock.

[PTEC]

Out of curiousity, can somone else with this software, and a known good MAF, take an air mass reading on a WARM engine, at idle, with the AC switched completely off.

I would definately recommed disconnecting the battery for a while. This while reset the adaption values in the DME. Reconnect the battery and start the car and it idle. Once the car begins regulated operation (aka closed loop) watch your STFT ( I hate using that term as it has no place in a Porsche since Porsche doesnt even use that system).

The most common reason for a car to go negative is a faulty air mass sensor. 90% of the time when an air mass sensor fails it reads high.

The most common reason for a car to go positive is an intake leak, and of those by far the most common is a hole in the bellows tube on your oil separator. When you have an intake leak, your indicated air mass will be low. The reason for this is your engine is getting air that isnt getting metered by the air mass sensor (aka unmetered or false air.)

Lastly it appears as though I was incorrect about the engine running lean. P1123 and 1125 indicate the DME has adapted as far as possible to the lean threshold, aka fuel trim numbers are negative. I made a false assumption based on my misreading the units the air mass reading was given in and the fact his LTFT was going positive. When I first started replying to this thread, I didnt see the units the airmass was measured Grams/sec. Then I reread the thread and I saw the conversions for G/s to kg/s. Using the conversion factor given, his indicated airmass is 19.224. Now I know I said his air mass reading looks low earlier in this thread, but now I'm saying I think 19 is high. I usually see about 14-16kg/s on a 996 at 1500 ft elevation, warm engine, no AC.

By the way, the source for all of my information is the Porsche Service Training P21 Fuel and Ignition systems book.

Edited March 22, 2005 by PTEC

[1999Porsche911]

The MAF should read below 6 g/sec on a fully warm engine and an idle of 700. This figure varies a great deal with just a slight change in engine speed and from car to car. My idle speed is 800 and I average about 7 g/sec. This is why it is important that you record the normal readings of the sensors for YOUR car before you have any problems. I have seen cars that have no problems running a high reading on the MAF and others that will generate a fault at a lower level. If you record the normal readings, then you are more abt to be able to identify the cause of the problem if one occurs.

A too low of a reading woud be below 4 g/sec. If you have modified your intake system, you reading may be on the higher side of the limit.

By the way, the source for all of my information is the Porsche Service Training P21 Fuel and Ignition systems book. may increase.

Now you understand why many Porsche Technicians can't properly repair their cars.

Edited March 22, 2005 by 1999Porsche911

[PTEC]

If a persons MAF is functioning properly then its going to read the same as another properly functioning MAF, the engine its installed in doesnt make it read any different. Its not like every MAF sensor just reads different out of the box. They are not snowflakes.

If the MAF is reading high then it is faulty. If its reading high and not causing an oxygen sensing fault then chances are you've got two problems working against each other.. such as a bad maf sensor and a vacuum leak.

By the way, the source for all of my information is the Porsche Service Training P21 Fuel and Ignition systems book.

Now you understand why many Porsche Technicians can't properly repair their cars.

<{POST_SNAPBACK}>

Yeah because the factory doesnt know anything about the cars they build..

Edited March 22, 2005 by PTEC

[1999Porsche911]

If a persons MAF is functioning properly then its going to read the same as another properly functioning MAF, the engine its installed in doesnt make it read any different.  Its not like every MAF sensor just reads different out of the box.  They are not snowflakes. 

If the MAF is reading high then it is faulty.  If its reading high and not causing an oxygen sensing fault then chances are you've got two problems working against each other.. such as a bad maf sensor and a vacuum leak.

By the way, the source for all of my information is the Porsche Service Training P21 Fuel and Ignition systems book.

Now you understand why many Porsche Technicians can't properly repair their cars.

<{POST_SNAPBACK}>

Yeah because the factory doesnt know anything about the cars they build..

<{POST_SNAPBACK}>

You gotta start thinking out of the box PTEC. Of course all MAF are set within the same tolerances. But lets walk you through this. What makes the same MAF read different from one car to another? Could be air filter condition. Could be engine compartment temperature. Could be the pattern layout of the intake screen. Could be difference in resistance in the main harness. Could be a slightly different idle speed.

You see, there are many variables in what determines the reading of the MAF and these variable are commonly different from car to car. That is why they establish a "range" for these sensors.

As far as the book you are using to get your information...if that is the book that states P1123 and P1125 means the engine is running lean, then I suggest you throw it out, because that information is wrong. Just because you read it in a book doesn't make it true.

Edited March 22, 2005 by 1999Porsche911

[Richard Hamilton]

Sorry. mumeh, I am butting in again, but I have the same interest in this problem as you.

PTEC and 1999Porsche911:

According to Loren, and the Workshop Manual:

27 Oxygen sensing, area 1 Cylinders (1 - 3), P1123 (Short to ground below lower limit/lean mixture threshold) - MIL is on, emission relevant fault

35 Oxygen sensing, area 1 Cylinders (4 - 6), P1125 (Short to ground below lower limit/lean mixture threshold) - MIL is on, emission relevant fault

Possible trouble areas:

- Fuel pressure too high.

- Fuel injector leaking.

- EVAP canister purge valve open.

Why haven't you recommended checking the fuel pressure? Cheap, but not easy if you don't have the right equipment.

mumeh - your MAF readings are within the specification of the diagnosis section of the manual. Idle is on the top limit (680 +/- 20).

If you find out where the EVAP C-P-V is, what it looks like, and if it is servicable, let me know.

Edited March 22, 2005 by Richard Hamilton

[1999Porsche911]

Sorry. mumeh, I am butting in again, but I have the same interest in this problem as you.

PTEC and 1999Porsche911:

According to Loren, and the Workshop Manual:

27 Oxygen sensing, area 1 Cylinders (1 - 3), P1123 (Short to ground below lower limit/lean mixture threshold) - MIL is on, emission relevant fault

35 Oxygen sensing, area 1 Cylinders (4 - 6), P1125 (Short to ground below lower limit/lean mixture threshold) - MIL is on, emission relevant fault

Possible trouble areas:

- Fuel pressure too high.

- Fuel injector leaking.

- EVAP canister purge valve open.

Why haven't you recommended checking the fuel pressure?  Cheap, but not easy if you don't have the right equipment.

mumeh - your MAF readings are within the specification of the diagnosis section of the manual.  Idle is on the top limit (680 +/- 20).

If you find out where the EVAP C-P-V is, what it looks like, and if it is servicable, let me know.

<{POST_SNAPBACK}>

If you read my posts, each and every one of those possiilities were mentioned. It is extremely unlikely it is fuel pressure as he has no symptoms other than the CEL. These sensors can be a pain in the *** to diagnose. Most often, the only method of identifying the problem is trial and error. Sensors are telling you that the engine is running rich and you should proceed with troubleshooting based on that fact. It is also unlikely that the EVAP system is open because of no other errors and engine operation. You have to look at the easiest and most likely things first.

If it was my car, the first thing I would do is put a 220 ohm resister inline on the #4 wire of the MAF harness, thereby reducing the voltage of the MAF and eliminating the possibility that it is reporting too much air flow. If the reported air flow was too high, then this should eliminate the lean threshold error of the O2 sensors. It's an easy test and also a permanent fix of an uncalibraded MAF.

Also, the specs for the idle for a C2 is 700 +/- 40 and his is fine.

Edited March 22, 2005 by 1999Porsche911

[Richard Hamilton]

1999Porsche911 - I hope I haven't offended you, as you clearly know a lot more about this than I do. I'm just saying that the MAF readings and idle are within specified limits. Are you saying it is SO critical, that it may be triggering the CEL when the MAF reading becomes close to the limit? You see, mine shows the P1123&5 codes, but does not trigger the CEL. Like mumeh's, it started with an 1123 code, and then developed into both codes.

I have attached the DME setpoints from my manual. It shows idle speed as 680rpm - maybe UK spec is different, in which case I should keep quiet.

I have also come to the conclusion that nobody knows where the EVAPCPV is :( I'm just being lazy really, trying to avoid stripping out a lot of bits I don't need to, and then finding out that it isn't serviceable. I find it strange that the manual gives this as an area to check - isn't it the kind of thing that would cause an air leak, making the mixture lean? Or maybe it is drawing fuel fumes from the tank, having an enrichment effect?

DME_Setpoints.pdf

[1999Porsche911]

The purge valve is in the front right fender well in front of the door.

[Richard Hamilton]

The purge valve is in the front right fender well in front of the door.

<{POST_SNAPBACK}>

Not according to the diagram in Loren's post (on page one of this thread). The EVAP canister purge valve (item 1 on the diagram) shows as being connected to the inlet manifold. The EVAP canister itself is in the front fender, behind the fuel filler neck, but it looks like there are tubes running to the back of the car, where the EVAP canister purge valve is located. The "Operating purge valve" is by the filler neck, but I guess this must be different.

[1999Porsche911]

My fault...I call that the EGR valve. That is a simple system to test. Electrically, you should hear it every few minutes when the engine is running. It makea a fast clicking sound as it is opening and closing. It connects to the cannister and to the throttlebody. The vacuum line to the throttle body has a one way valve on it so flow is only in that direction. The line to the canister should be able to hold vacuum. If not, you would look at the Operating Valve in the fender well. Possible loose ventilation lines.

The only fault in the EVAP valve that would not cause another DTC would be a vacuum leak between it and the throttleody. All other defects should be reported to you either with a fault code or engine problems. I have never seen one of these valves fail.

Edited March 22, 2005 by 1999Porsche911

[PTEC]

Why haven't you recommended checking the fuel pressure?  Cheap, but not easy if you don't have the right equipment.

mumeh - your MAF readings are within the specification of the diagnosis section of the manual.  Idle is on the top limit (680 +/- 20).

<{POST_SNAPBACK}>

First off, he checked fuel pressure already.

Secondly, do the math. His reported air mass is 5.34 g/s. Converted to kg/h thats 19.224. Too high if you ask me AND out of spec according to the pdf you posted.

Also the purge valve is not servicable. Pretty much all it is is a one way valve with a line coming from the carbon canister and runs to the intake manifold. The valve is electronically opened (PWM) by the DME. The DME also monitors changes in the exhaust gas resulting from the purging of the canister and compares the duty cycle of the vavle to the change in the exhuast gas. If the results do not match up, then a fault is generated and the MIL light is illuminated. These valves do fail occasionally but when they do a DME fault is registered along with it (which he doesnt have). 1999Porsche911 gave pretty much the only way to test them. With the valve electrically unplugged it should hold vacuum.

While I have never changed one myself, I seem to remember the Evap purge valve (sometimes called by Porsche the Tank vent valve) being in the engine compartment right above the oil filler tube.

Edited March 23, 2005 by PTEC

[PTEC]

If a persons MAF is functioning properly then its going to read the same as another properly functioning MAF, the engine its installed in doesnt make it read any different.  Its not like every MAF sensor just reads different out of the box.  They are not snowflakes. 

If the MAF is reading high then it is faulty.  If its reading high and not causing an oxygen sensing fault then chances are you've got two problems working against each other.. such as a bad maf sensor and a vacuum leak.

By the way, the source for all of my information is the Porsche Service Training P21 Fuel and Ignition systems book.

Now you understand why many Porsche Technicians can't properly repair their cars.

<{POST_SNAPBACK}>

Yeah because the factory doesnt know anything about the cars they build..

<{POST_SNAPBACK}>

You gotta start thinking out of the box PTEC. Of course all MAF are set within the same tolerances. But lets walk you through this. What makes the same MAF read different from one car to another? Could be air filter condition. Could be engine compartment temperature. Could be the pattern layout of the intake screen. Could be difference in resistance in the main harness. Could be a slightly different idle speed.

You see, there are many variables in what determines the reading of the MAF and these variable are commonly different from car to car. That is why they establish a "range" for these sensors.

As far as the book you are using to get your information...if that is the book that states P1123 and P1125 means the engine is running lean, then I suggest you throw it out, because that information is wrong. Just because you read it in a book doesn't make it true.

<{POST_SNAPBACK}>

I agree there are tiny differences such as the air filter, but even then thats going to make the air mass read low, not high. Two of the same cars under the same condtions should read the same air mass maybe give or take a .25 kg/h. It sounded to me like you were trying to say that two different air mass sensors would have two different readings just because they were different sensors. If thats not what you were saying then I misunderstood you.

If you read my post, you should have seen I already admitted I was wrong about the engine running lean. I made that assumption based on the fact that I didnt know the conversion for grams/s (which is the aftermarket unit for airflow) to kg/h (which is what the factory tools report). I saw what I thought was 5kg/h (way low) of airflow and postive fuel trim. I misread the picture he posted... newflash: I'm not perfect.

Ill refrain from posting any other ideas about what is wrong with his car until he comes back and posts data after the battery reset.

[Richard Hamilton]

First off, he checked fuel pressure already.

Secondly, do the math.  His reported air mass is 5.34 g/s.  Converted to kg/h thats 19.224.  Too high if you ask me AND out of spec according to the pdf you posted.

PTEC

1) I have read through this thread again, and I don't think mumeh has checked his fuel pressure. I checked mine.

2) I did do the math. The spec is 17kg/hr +/- 2,5, which means his his reading of 19.224 is below the top limit of 19.5, therefore within spec.

Thanks for the location of the valve - I'll give it a try over the weekend. I too am very sceptical about this valve being the problem. It's a bit confusing because according to the diagram there are two purge valves. If the one in the engine compartment is just a one-way valve then surely it can't be operated by the DME? I guess I'll find out when I reach it.

My problem isn't as serious as mumeh's as I just get the error codes, not the CEL. However, it's probably only a matter of time. I apologise for hijacking his thread, but we have the same goal. Like him, I am just trying to eliminate the easy-to-fix items before I get into buying expensive sensors and injectors.

I appreciate all the advice you guys are giving. It has given me a much better understanding of what is going on.

[PTEC]

The purge valve in the engine compartment IS operable by the DME. Its a electromagnetic one way valve. There is a line to the carbon canister on one end and on the other end the line goes to the intake right near the throttle body. Also, there is an electrical connector on the purge valve which goes to the DME. You'll find the evap canister purge valve right above the alternator.

The other valve in the diagram is the shut off valve (or operating air valve or even purge air valve). Its of the same design as the tank vent valve but is located in the carbon canister. This valve is used to seal off the tank system when its doing its leak test. Opening this valve allows fresh air to be drawn into the carbon canister while the system is being purged.

Edited March 24, 2005 by PTEC

[D_Man]

Hey what was the out come on this topic. This is better reading than a dime store novel. I have the same codes BTW. I think porsche / bosch have made millions on the this MAF component.

D

[mumeh]

I owe everybody here a big THANK YOU for their input. I was not able to tinker with the car for a while, and I suffered bigger problems than the CEL, as my battery was starting to go south. I looked at this battery/alternator problem for a bit (charging with a charger, observing behavior, etc.) and then just decided to take the plunge and get a new battery. Not only was my battery problem solved, but lo and behold, no CEL! I drove the car with no CEL for weeks, and thought that I had solved the problem. I began to view myself as "Master of the Technical Universe." Why, my arm was almost getting sore from patting myself on the back! I thought about writing a post here about the value of keeping an open mind when troubleshooting complex systems, and how many of us fall into the trap of assuming certain problems to be more technical than they really are, blah, blah, blah.

Happy to have a car with no CEL and problem resolved, right?

Wrong.

CEL just went on again after a month or so of light driving. Yep, P1123. I guess I'll disassemble the entire car now looking for the mythical evap canister (maybe it will be filled with magic candy!) now based on Loren's diagram (thank you Loren). Drats. I've been humbled, and my "Master of the Technical Universe" title has been ripped from me, and alas, now I am just a guy with car with a glowing CEL. Fame is indeed fleeting.

Back to square one.

As usual, advice welcome.

Edited August 6, 2005 by mumeh

[golfhom]

I owe everybody here a big THANK YOU for their input.  I was not able to tinker with the car for a while, and I suffered bigger problems than the CEL, as my battery was starting to go south.  I looked at this battery/alternator problem for a bit (charging with a charger, observing behavior, etc.) and then just decided to take the plunge and get a new battery.  Not only was my battery problem solved, but lo and behold, no CEL!  I drove the car with no CEL for weeks, and thought that I had solved the problem.  I began to view myself as "Master of the Technical Universe."  Why, my arm was almost getting sore from patting myself on the back!  I thought about writing a post here about the value of keeping an open mind when troubleshooting complex systems, and how many of us fall into the trap of assuming certain problems to be more technical than they really are, blah, blah, blah. 

Happy to have a car with no CEL and problem resolved, right?

Wrong. 

CEL just went on again after a month or so of light driving.  Yep, P1123.  I guess I'll disassemble the entire car now looking for the mythical evap canister (maybe it will be filled with magic candy!) now based on Loren's diagram (thank you Loren).  Drats.  I've been humbled, and my "Master of the Technical Universe" title has been ripped from me, and alas, now I am just a guy with car with a glowing CEL.  Fame is indeed fleeting.

Back to square one. 

As usual, advice welcome.

<{POST_SNAPBACK}>

[golfhom]

HI;

I tried the battery disconnect couple of times. The light will go out until you drive your car for a week or ten days. It comes on again after the computer does the calculations, then the CEL Will come back to haunt you again.

I took Loren's advice and took it to the "Real Mean Machenic". He replaced the Mass Air detector and I passed smok. I been driving for six months since and no CEL. Cost me $800. I feel too old and stiff to work on the cars now. So I pay for the pleasure of driving. Golfhom

[ivanuf]

I had the same codes on my 996 for a while. I changed the air filter, use an electrical contact clearer for the MAF sensor, cleaned the throttle body. But the code kept coming up. The dealer diagnosis was. Mass Air Flow part#996-606-123-00 also (2) vacuum hoses p# 000-0043-205-01 and idle-speed p#996-606-160-01. Will see it they were right. Will keep you posted.

[mumeh]

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.

I wanted to close the loop here on the orginal problem. I checked a lot of different things, and after finally checking for vacuum leaks I took the plunge and bought a new MAF sensor. I have waited about three weeks and driven the car quite a bit, and no CEL. Also, throttle response is improved. The car has never made it more than a few days with a CEL after a reset. I consider this problem SOLVED. Thanks to everyone who chimed in, and I hope that my trials and tribulations might help someone else with similar problems. Thanks again, everyone!

[Izzy]

Mumeh,

If I follow this thread correctly, you changed both the MAF and the O2 sensor for banks 1-3?

I am asking because I have the same code, P1123 only. At 60K miles I had the MAF code (not 1123), cleaned it (for the last time) and could not clear the MAF code. So changed it and all was clear.

500 miles later, I am getting P1123. Loren had mentioned it was a close possibility due to mileage that the 02 sensors would be next.

Just wanted to verify that was your fix, and the CEL is gone for good? Thank you

Izzy

Edited July 26, 2006 by izzyandsue

[mumeh]

Izzy,

Hope I can help here. I did indeed replace both the MAF and the O2 sensor for banks 1-3. The difference is that, for me, the P1123 code did not actually lead me to the real probem right away. I replaced the O2 sensor first, and the CEL was quickly back on again, indication that the O2 sensor was, at best, not the sole source of the problem, and likely was in perfect shape. What I have learned from my experience and the many other posts here is that the P codes give you hints as to the normal operating parameters that are being exceeded, but they do not always point you directly to the source of the problem. That said, it sounds like you may have a different problem, since you already have a brand new MAF. My experience with other cars tells me that oxygen sensors are a good first place to look for things like this (especially after 50K+ miles), and since this sensor is not horribly expensive and it is easy to replace on the 996 (no more than 15 minutes, including jacking the car for better access and hand-washing), this should probably be your next move. If it makes you feel any better, rest assured that, given your error code and the history of the car, this is the FIRST thing any mechanic would look at with a P1123. With the MAF on my car, I hesitated after first replacing the O2 sensor because I did not want to "throw good money after bad," but in the end since I have replaced it, all my CEL problems have gone away and I am back to driving the car with confidence in its reliability. Easily worth the price.

Hope this helps.

Martin

Mumeh,

If I follow this thread correctly, you changed both the MAF and the O2 sensor for banks 1-3?

I am asking because I have the same code, P1123 only. At 60K miles I had the MAF code (not 1123), cleaned it (for the last time) and could not clear the MAF code. So changed it and all was clear.

500 miles later, I am getting P1123. Loren had mentioned it was a close possibility due to mileage that the 02 sensors would be next.

Just wanted to verify that was your fix, and the CEL is gone for good? Thank you

Izzy

[Izzy]

Martin, thank you very much for your quick reply. Got the sensor on order.... Bank 1-3 is the left side, and the sensor before the cat. Just verifying. Thanks again

Izzy