replacing parts, still not fixed
 

I have a 2000 Boxster S, 43,000 miles. check engine light came on and the code is a P1126. I have replaced the O2 sensors, both banks, MAF, and still not fixed. I have sprayed starter fluid on many parts looking for leaks to no avail. The range on the MAF measurement did get better with new MAF. What next?

Included a PDF with OBDII data. First data is idle, concern with STFT is N/A and 99.2. Following data is at different revs.

P1126 That is usually = a lean condition on Bank 1 .The plug color may confirm. Usually a vacuum leak.
Make sure you used the correct part number O2 sensors .Easy to mistakenly use the wrong ones.Cross check the Porsche part code if you Interchanged to Bosch.Ditto the MAF.
Check for oil filler tube crcks(common) and vac hoses/connections

I purchased the O2 sensors from Pelican and stayed with Bosch, figured they had it right.

Check for a vacuum leak in the evap system and the auxialary air injection system. Very hard to diagnose so don't start throwing parts at it because you can easily spend thousands without finding the problem


First thing is to use a durametric to make sure you have a vacuum leak. If the maf spec is significantly below spec, 15, then you probably have a vacuum leak.

Any thoughts on the N/A and 99.2 %?

I found a super way to find a vacuum leak, or completely rule one out while trouble shooting a SAI problem. It's a bit involved, but bullet proof.

First roll the engine to TDC. (That's the first part of involved)

Then pull the plastic intake tube off the throttle body and slip a 3 mil piece of plastic between the tube and throttle body, put the tube back on the throttle body (the second part of involved)

Pull a vacuum hose off the intake manifold and blow into it. The manifold should hold air. It should feel like you are trying to blow up a hot water bottle. When you release the air, it should blow back out the hose. Holding my finger over the hose, my intake held pressure for about two minutes. Pretty impressive.

If your manifold leaks down, it's time for a cheap cigar. Light the cigar, take a deep drag (don't inhale though!), and blow that lovely smoke into the intake manifold. Doing this in the dark with a flashlight to find the smoke leaking out works well. Small as the leak was, I saw a tiny bit of smoke coming from the valve in the resonance tube

This also works on the lower end using the oil fill tube, electrical tape, and the corner of a 3 mil trash bag (dipstick tube might work as well, I didn't actually use that).

Best of luck.

I agree but please use a decent cigar. It is a Porsche after all.

We chased a vacuum leak for a year before the power brake booster fully failed and revealed that it had been leaking a long time. Vacuum is used all over and the lots of tubes and hoses. A smoke test will not reveal the booster. You need to test the booster separate.

We've had some epic Threads on Vacuum Leaks. All I concluded was that a single tool may never find the leak(s). So I have a Smoke Machine, a Propane wand, Carb Cleaner aerosol and use them all.
Some times just disconnecting+plugging small suspect sections is the best technique.
Some very talented guys here have spent many frustrating hours tracking vacuum leaks.Only a few gave a detailed,helpful confessional. I guess because when you find the leak it seems so simple?
You need to be absolutely logical and methodical and use the Vacuum diagram.
By far the best Thread I recall was Jamesp.Try Search?
Good Luck

I tried the cigar trick and seemed to find a leak from the throttle body gasket. Replaced the gasket but still have issue. Took to a shop and had a smoke test, unfortunately it passed with flying colors. One of the mechanics thought he heard an exhaust leak and said the gasket on the manifold to cat sometimes goes, does that sound like a possible solution?

Hi,

yes, if you have a faulty gasket on the manifold, the o2 sensors detect too much o2 and this will generate a DTC. Also P1126 That is usually a lean condition on Bank 1. Means too much o2. So i would recommend to check all gaskets on bank 1 between engine and the o2 sensors. If there is an exhaust leak on the manifold to cat, this will be the problem.

Regards, Markus

P1126 is on bank 1 normally for:


1. Intake system is leaking
2. Fuel pressure is too low
3. One or more injectors is failing
4. Volume of fuel delivery is low


I would suggest running fuel trims, check fuel pump for correct delivery pressure and volume. I would also pull the plugs on bank 1 looking for one that is sooty or black and wet.

The fuel trims are in the attachment in the first post

Your short term fuel trims are out of spec by a mile. For example, an "in spec" "long term" FRA trim would be around 1.0 +/- 0.1 and an in spec "idle range" fuel trim would be in the range 0.00 +/- .2. There is definitely a leak issue somewhere, or your fuel system has an issue.....

System description

The adaptation values can be read out with the Porsche System Tester 2.

The key to the adaptation values is as follows:

RKAT = Adaptation in range close to idle

Cylinders 1 - 3

RKAT2 = Adaptation in range close to idle

Cylinders 4 - 6

FRAU = Adaptation in lower load range

Cylinders 1 - 3

FRAU2 = Adaptation in lower load range

Cylinders 4 - 6

FRAO = Adaptation in upper load range

Cylinders 1 - 3

FRAO2 = Adaptation in upper load range

Cylinders 4 - 6

FR = Oxygen sensor for cylinders 1 - 3

FR2 = Oxygen sensor for cylinders 4 - 6

My understanding of range 1 and range 2 is that range 1 is the reading before the DME adapts the mixture and range 2 is after adaption.

thanks jfp, i've wondered about the frao readings - i had presumed that the frao values were reserved for turbo cars that had wideband o2 sensors and returned afr readings at high load.

reading the report attached, i note that you are returning a 'n/a' value for bank one sensor two. it does report a voltage value, however, so presume the sensor is working. my understanding is that the second sensor is the post catalytic o2 sensor and only used to confirm operation of the cats and not to trim air fuel ratios. so that begs the question as to why bank two is returning a trim value for sensor two. probably tied to the fact you are using a non-porsche obd tool to read the data?

so, as jfp notes, you have two trim ranges - rkat (idle) and frau (low load). factory ranges are +/- 4% for rkat and 0.7 to 1.32 for frau. again, the obd data you have looks to be generic so uncertain what trim info is being presented. the trim values presented could be in range if frau value, but are still high. if idle (rkat) then too high. note that a vacuum leak will be more obvious at idle than at low load - a car driving down the road uses a lot more air than an idling car, so a vacuum leak will show more at idle (ie, 1 part to 50 vs 1 part to 500). a bad fuel system will more likely show up under load where the requirement to provide fuel is higher than at idle. realise that you can get a lean condition (ie your current code) with too much air (vacuum leak - problem at idle) or not enough fuel (fuel system issue - problem under load). perhaps get a durametric or pst2 on your car?

It would appear my efforts have paid off. I believe replacing the gasket on the throttle body sealed it. A smoke test revealed that there was no vacuum leak and taking to a well know Porsche garage validated the numbers are good. Thanks everyone for reviewing my information and providing analysis/suggestions. Jim