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Code P1124, P1126, P0446

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I have a 1999 Boxster (2.5 L), 5 speed manual, USA which I bought with 67,500 miles five months ago. Car sat unused almost a year before I bought it. I started driving it a few weeks ago sporadically when the weather permitted. Last week the check engine light came on and I brought the car to Pepboys to read the on-board diagnostics. Code P1126 came on. I read my Bentley book and read through articles here and checked air filter. It was badly clogged up so I replaced it and had code reset. A few days later check engine light came back on and this time it reads: P1126, P1124, P0446. What does this mean? How can I fix this? I will fix it myself if I can find out what to do. Can you assist me? I want to drive the car, but am afraid that if I don't correct these problems soon I will make matters worse if I continue driving the car. HELP!! Many thanks.

Edited by questforbalance
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As you drive your Boxster it is continually running tests on the emission system to determine if the car is meeting emission standards. As each test module (you will find a list of them in the Bentley book) reports back to the ECM (located in the rear trunk behind the carpet on the left side) if anything is either not reporting properly, is on the verge of failure or has failed a fault code is stored for that particular module. After so many run cycles if those fault codes continue the CEL comes on. Most CEL issues are emission control issues and all three of yours are. Chasing them down and fixing them can be both time consuming and expensive unless you can do some of it, or all of it yourself. I'm having similar issues with my '99 in that I have fault codes P1126 & 1130, P0130 & 0150 that have caused my CEL to come on. Because one affects the other just randomly replacing parts might not necessarily fix the problem and costs $$$ that may not have needed to be spent. Start at the front of the car and work back, check your battery terminal connections (yes a loose battery connection can cause intermittent shorts), check your ECM to see if there are any loose connections or that rodents haven't gnawed on wires causing them to short (you said the car had been stored so this could occur). P0446 is an EVAP valve malfunction so you might want to test it (or have it tested) to see if it is malfunctioning or just needs to be purged ( have you been topping off the tank when you fill it?). The next step would be to test the fuel pump voltage to see if it is operating correctly (P1124) then your O2 sensor to see if the electrical connection is solid (hasn't been gnawed on) and is operating properly. Testing is less expensive than parts and will help you isolate the issue that is the real cause of the chain of events that is happening right now that is causing your car to run either too rich (any smoke regularly coming out of the tail pipe?) or too lean. Good luck Boxster codes are a good and bad thing as they can pre warn you of a costly failure but can drive you crazy trying to isolate the main cause of the faults.

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Thanks for the advice. I changed my air filter, cleaned my MAF sensor, put a bottle of techron in gas tank, cleared codes and drove car for five days and codes have not reappeared. Glad to say, that for the moment my problem is resolved. Thanks. great forum. Hope my experience helps someone else.

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  • 1 year later...
  • 5 months later...

Bump! As I have these codes now too. :(

Started with P1126 on my '99 Boxster a couple weeks ago. Sometimes upon starting, the RPMs will rev up and down then stall. Occasional strong exhast smell upon shutdown and getting out of the car. I've searched for this code (originally searched P1126, now searched P1126, P1124, and P0446) on this forum and a few others and followed the troubleshooting steps the best I could without having to buy new parts yet:

1) Cleaned MAF

2) Visually checked for leaks (looked for cracks, loose connections, etc)

3) Oil filler cap o-ring appears intact and cap isn't difficult to remove when engine is running

4) Gas cap is new

5) J tube and intake tubes have a slight amount of oil in them, more than a mist, but not dripping wet

Yesterday, filled up the car, drove 1.5 miles and parked overnight. Next day, get code P0446. Attributed to the possibility that this new code was due to a full fuel tank. Reset codes and drove 30 miles roundtrip with a five hour stop in between. On the last mile coming home, got a CEL. Codes this time are P1124 and P1126.

I was ready to order a MAF, but now that I have P1124 and P1126, from what I've been reading, more likely an intake air leak and not a MAF?

Edited by bar10dah
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Okay, so here's some contradicting info.

Durametric (Bosche DME Motronic 5.2.2) says:

P1124 Oxygen sensing range 1 Cylinder (1-3)

P1126 Oxygen sensing range 1 Cylinder (4-6)

Bentley (Table b. DME M 5.2.2 DTCs) says:

P1124 Fuel pump relay output stage

P1126 Oxygen sensing adaptation, lower load range, cyl. 1-3

What gives?

UPDATE: I asked Durametric which was accurate. They said their information is accurate and the Bentley is wrong. Given that quite a few posts on Boxster forums also confirm that the Durametric data is correct, I'll go with that! Guess I'll just pen & ink change my Bentley. :)

Edited by bar10dah
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  • Admin

From Porsche OBD II Manual:

P1124 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold

P1126 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold

Possible causes:

- Intake air system leaking.

- Fuel pressure too low.

- Volume supply of fuel pump too low.

- Fuel injectors fouled

What the DME computer is saying is that it thinks the mixture is too lean (too much air) and that it can not compensate by making the mixture richer (adding fuel).

9 times out of 10 this is an air leak.

An air leak can be a loose or damaged intake hose, a cracked oil filler pipe, or a bad AOS.

Do a search here for tips on how to check for these,,,

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Thanks Loren, I did find your exact reply here in many other posts about these codes. Thanks for always being there to help everyone!

I've done the troubleshooting that I could, based on the other threads. I'll play around more with the oil filler tube to see if I can tell if there are any leaks there. In the mean time, a new MAF is on it's way. I'll order new brass fittings/o-rings (and try to find a tester) for the fuel test if that doesn't pan out. Otherwise, I'll try to find someone local that can do a leak test.

Even though my P0446 hasn't reappeared yet, I'll look up how to troubleshoot the EVAP system. I tried activating it with Durametric, but couldn't hear any clicking/engaging, so perhaps I do have a problem with that system. Or, I just couldn't hear it working.

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  • 2 weeks later...

Replaced the MAF, replaced the AOS, also replaced the oil filler cap and gas cap. I'm still getting P1124 P1126 and P0446. I couldn't find any vacuum leaks (without a smoke machine) in any hose I could reach. And I can't find any cracks in the oil filler tube (where do cracks usually form on the oil filler tube? and, which half of it, the half that attaches to the engine or the half that goes into the trunk area and has the flex part on it?).

I really didn't find a good thread that talks about testing the charcoal tester. How do I go about doing that?

Also, for others with P0446, it was recommended to blow compressed air through the vent line. Anyone have a snapshot of the line I need to do that to? I'm just not sure which.

I've documented my troubleshooting process here, if you need more information.

I sure could use some help on this. I've read through countless posts here and on other forums and have done most what I could and still no resolution to my problem. I'm just out of ideas...

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  • 5 weeks later...

Replaced the MAF, AOS, lower AOS tube, oil filler cap, gas cap, both oil filler tubes in the engine bay, all those 4mm vacuum lines under the passenger side air intake manifold, the SAI shut-off valve, both pre-cat O2 sensors... I still have P1124 and P1126.

As far as I know, the only thing I haven't checked is the fuel system. Although, I do have a new fuel filter. Is there anything else I can check?

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Replaced the MAF, AOS, lower AOS tube, oil filler cap, gas cap, both oil filler tubes in the engine bay, all those 4mm vacuum lines under the passenger side air intake manifold, the SAI shut-off valve, both pre-cat O2 sensors... I still have P1124 and P1126.

As far as I know, the only thing I haven't checked is the fuel system. Although, I do have a new fuel filter. Is there anything else I can check?

Have you done a smoke test to see where the leak is?

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Short answer, no.

I took it into Porsche a couple weeks ago, requesting a smoke test. After about 20 mins, the mechanic came out and brought me back. He said he can hear the leak coming from under the passenger side intake. He then started the car and let it idle. He then turned the oil filler cap to open it just a bit, and immediately there was a hissing sound. Sticking my head in the engine bay a bit, I could definitely hear the hissing sound more pronounced from the area the mechanic said it was in. So the mechanic said I didn't need a smoke test, just pull out the passenger side intake and look for either a cracked line or test the valves under there to make sure they're operating correctly. I didn't find any broken lines, but I did replace all the 4mm lines anyways. The check valve (#22) going to the intake boot was good. I couldn't test the shut-off valve (#15), so I just replaced it. The only valves I didn't replace is the change-over valve (#18) and the check valve (#10) that goes into the cylinder head. I'm going to order those two tomorrow and see if that makes any difference. If not, back to the Porsche dealer for the smoke test.

In this photo, I replaced #15, 19, 20, 21, 25, 26, and 27. This is the area the sound is coming from, all under the passenger side intake manifold. 11, 13, and 16 don't appear to have any cracks. 9, 10, and 15 are next on my list to replace.


The colored arrows was just me trying to learn how the air flow worked. I determined the red lines to be the vacuum, originating from the air intake boot. And the blue lines the ambient air from the Secondary Air Injection system.

Edited by bar10dah
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#9, 10, and 18 were ordered today. Should be in on Wednesday.

Drove my Boxster 850 miles today. CEL only came on once, when I had to get off the interstate 173 miles into my trip and do the stop and go dance through a four way stop sign. It was my dreaded P1124 and P1126 again. Reset the CEL and got back on the interstate. Went the rest of my trip without the CEL coming on again. Seems like the only time those codes get set is when my car is at idle. After resetting my CEL, went 680 miles without a single CEL. Only pulling off the interstate to gas up or grab food, with minimal idling.

Edited by bar10dah
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1,667 total miles from my two day trip. CEL only came on when driving around in the city. All highway driving results in no codes. Strangely enough though, driving through an hour of stop-and-go Chicago traffic didn't cause the CEL to come on.

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Not #9, but #10 you mean, the check valve?

Good work if that solves it.

Crap, yes! Thanks for catching my error. The part pictured is #10 from the diagram in post 12, above.

Still not sure if this was the root cause of my problem. Putting everything back together, thought about another way to route one of the vacuum lines, to try to keep it from rubbing/resting against the hot engine block. As I try to pull the connector off the vacuum reservoir (part #16), I guess I applied too much lateral force and snapped the nozzle off. New $33 vacuum reservoir ordered from the local dealer and will be here on Thursday. Depending on the rain we're forecasted for, I may have to wait until the weekend to install it and get the car back together. I'll post my results, either way. After reading probably close to 100 threads while trying to research my problem, I hate it when someone doesn't come back to post if what they tried fixed the problem or not. :(

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Have you tested fuel pressure & quantity, also fuel pressure regulator?

Easy to do, just get the right tool for the job!



I can try that! Thanks for the tip. If I can't find the Actron fuel tester (from the first link) locally, I can stop by Harbor Freight and pick up the A/C tester (from the second link). I did find the Actron CP7838 fuel tester at Sears for $40.04 plus $7.75 shipping, so I can go that route too, but would need to find the correct fitting. Appears the only place to get 0180-000-1299, the right angle large schrader adapter, is from Actron for $3.32 plus shipping. I'm guessing it's better to buy the Actron vs the HF A/C tester?


What's your intake vacuum? The gauges are cheap, and it's easy to pull an intake hose to test with.

I'm guessing the vacuum gauges can be had at a local auto parts store or Harbor Freight? I seem to remember reading a post where someone fabricated an old oil filler cap to fit the connection for the vacuum tester. Glad I kept my old oil filler cap, if I have to do the same thing.

Edited by bar10dah
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The harbor freight A/C manifold test kit works for me and very easy to use, everything you need in the one box to test fuel pressure. I think I got it for $30 or $40 a long time ago, it was on sale and I used a 20% coupon. THe price might have gone up by now.

Yes the harbor frieght vacuum gauge works fine. Since you've eliminated a lot of stuff for those codes, I haven't seen a test for fuel pressure with engine off or engine on idling.

You can test crankcase vacuum, but you should probably actually test intake manifold vacuum. If your reading is low, it might indicate a vacuum leak or otherwise, and I can't recall if you've done a smoke test yet or not.

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Went to Harbor Freight today. The A/C manifold test kit was $49.45 with tax. Unbeknownst to me, my 20% off coupon expired five days ago. :( So I decided to come home and order the Actron CP7838 from Sears, as it was only $42.34 with tax/shipping after a coupon I had. Also added an Actron vacuum gauge so I can check the intake manifold vacuum.

I'll order the correct fitting directly from Actron tomorrow, as I wasn't able to find it online.

I also ordered the fuel pressure valve from Pelican. Most of what I've read about testing the fuel pressure at this port says you must replace this valve. Although, I've seen a few posts where the valve wasn't replaced and didn't cause any issues. However, I don't want to take any chances with fuel + hot engines.

I should get the parts in a week or two and will report back what I find. From the links above, fuel pressure should be 43.5 to 58 psi when pump is activated by Durametric? How about intake manifold vacuum? How many inches of vaccum am I looking for?

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