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I've been trying to diagnose a code P1126 for a while now. I have found lots of post and articles about this code but it seems to almost always be accompanied by a code P1124.
Typically a bad MAF or intake leak. there are no obvious leaks (although I have yet to do a smoke test). and I am think (could be wrong) that if the MAF was bad, it would surely though both codes.

has anyone else had just the one code thrown, is there any significance to it or just "one of those things" that it is not throwing a P1124 along with it?

P1124 is oxygen sensing adaptation range 1 (cyls. 1-3) at enrichment limit.
P1126 is oxygen sensing adaptation range 1 (cyls. 4-6) at enrichment limit.

One or both of these codes are usually caused by an intake air leak - more air is getting into the engine than measured by the MAF. This results in a lean condition (more air than fuel) so the ECU incrementally adds fuel but reaches the limit before eliminating the lean running condition.

Alternatively, these codes can also be caused by not enough fuel, i.e., fuel pressure too low, fuel injectors contaminated, or volume supply of fuel pump too low.

As you have already figured out, all of these conditions should typically result in the same lean condition on both engine banks so one would expect both codes to be present. However, it is possible to have an air leak only on one bank so you'll want to work your way through the list of standard causes.

However, after some research, I found the link below about the AOS resulting in only the P1124 code. Might be worth checking your AOS also. Also check the seal of the oil filler cap itself while you're testing the AOS.

Let us know what you find.

Check engine light on, Code P1126 on 99 Boxster - 986 Series (Boxster, Boxster S) - RennTech.org Community

Quote:

Originally Posted by thstone (Post 447306)

After reading that link I am thinking it is the AOS. I just replaced it about 9 months ago, but used the cheaper version on Pelican. there is some oil in the J tube, the throttle body and the resonance tube. not much, but a little. no smoke on hard acceleration like before, but I am guessing that any oil in those areas means bad AOS.

so my first task will be a smoke test for vacuum leaks, then replacing the AOS.

Quote:

Originally Posted by kiyakubuke (Post 447333)

actually, looking back at my orders, it looks like I did get the OEM Porsche brand AOS. 2 year warranty so I guess I need to get a new one.

Another good indicator of a failed AOS is excessive vacuum on the oil filler cap. If it's good it should be easy to remove while idling and a bit of vacuum is normal as well as a rough idle when removed. When my failed I could just barely remove the cap.

So I replaced the AOS, cleaned out the throttle and resonance tube, replaced the oil filler tube and pipe. the Oil leak is gone, but I am still getting the p1126 code. car runs fine
I hooked up my OBD2 BT running torque Pro, my short term fuel trims long term are 31.25% for bank 1 and 25.78% for bank 2.
and my O2 volts are .11 for bank 1 and .65 for bank 2 varying up to about 4 max.

I haven't been able to find what the specs are for the 986 3.2 engine, but for other cars I have heard than anything over 10 for the fuel trim long term will set off the CEL.

Quote:

Originally Posted by kiyakubuke (Post 451368)

, my short term fuel trims long term are 31.25% for bank 1 and 25.78% for bank 2.
and my O2 volts are .11 for bank 1 and .65 for bank 2 varying up to about 4 max.

I haven't been able to find what the specs are for the 986 3.2 engine, but for other cars I have heard than anything over 10 for the fuel trim long term will set off the CEL.

This link may help your diagnosis? Admire your perseverance !
Fuel Trim?

Quote:

Originally Posted by Gelbster (Post 451381)

This link may help your diagnosis? Admire your perseverance !
Fuel Trim?

thanks for the link. good info. funny thing is that, according to the info I am getting from torque Pro, both my bank 1 and bank 2 should be throwing codes. I am pretty sure the p1126 code is for bank 2, which is actually lower than bank 1. and p1133 is for bank 1. but I'm not getting that code.
Also, I am wondering what the time frame is for the long term average. my short terms readings are well within spec at all rpms and speeds. so I am wondering if I just have not driving it enough since replacing the AOS and Oil pipe to lower the average.
so guess I'll drive it as is for a bit, see if it clears. if not, time to build a smoke machine and check for a leak.

replaced the MAF

replaced the MAF sensor today. the first thing I noticed was that when I reset the fault codes, the engine did not start running rough like it had previously. good sign. so now I need to drive it a few hundred miles to see if the codes return. I am optimistic.

ok, so the car still runs fine and the LTFT readings for both bank 1 and bank 2 are staying within spec 0 +/- 5 (they were hitting 35.1 before). however, the CLE came back on with the same codes. p1126 and p1133. I was under the impression that it was the LTFT readings being to high that was throwing these codes. does anyone know what other readings could throw them? O2 voltage etc..?