Boxster with P1124 and P1126
 

Hey guys,

I have a 99 Boxster 2.5L which has has these two codes for a while. I've been reading a lot on these codes on this forum and other places and I have an idea of things I might want to try next but I wanted to brainstorm what would make the most sense with people here.

Here are some details:

• 90k miles. Engine runs great, constant power delivery, no rough idle.
• P1124 and P1126 always come back within a day after clearing codes, always in pair
• Engine seems to run hot if the car is left running at idle, the temp indicator eventually crosses the middle of the gauge
• The car fails smog with high NOX
• I've notice some oil in at least one of the spark plug tubes when I changed the spark plugs a few months ago. It was for the cylinder closest to the cabin on the driver side (cylinder 1?)
• No apparent excessive smoke

Here's what I've tried so far:

• Replaced MAF. This didn't fix those codes but made the car behave a lot better (lot smoother power delivery, engine seems to run stronger)
• Replaced gas cap. Didn't fix the issue.
• Opened the oil filler tube cap with engine hot. RPM dropped and recovered shortly after as expected.

For next steps, here's what I'm thinking:

• Replace AOS? The oil in the cylinder seems to indicate that the AOS might be bad, but the car doesn't smoke and the old MAF didn't seem particularly dirty when I replaced it so I'm not too sure if that would help. Any thoughts?
• Air leak? The car seems to be running hot which might be due to an excess of oxygen due to an air leak, making the mixture too rich. This could be causing the high NOX causing the smog to fail too since the car is too hot. The car responded well to removing the oil filler tube cap so I'm not sure if that is the problem. I'm also not so sure how to diagnose that as a DIY-er - I've read about a few techniques like using soapy water but I'm not sure where to start.
• Weak fuel pump? The engine is running lean so I either have too much air or not enough fuel. Any tips on how to diagnose the fuel pump?
• Replace spark plug tubes? Maybe one of the tube is cracked and is causing the oil to leak through? There's no white smoke though so I'm not convinced of that.

Do you have any additional diagnostic steps I could run? I have an OBDII reader that I can consult. I've read that air flow should be between 13-15 for example. Is that right? Any other values I should monitor?

Thanks so much for your help!

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

P1126 is same error for cyls. 4-6.

If DTC P1124 or P1126 logged then fuel/air mixture is too lean.

Possible causes: intake system leak; fuel pressure too low; fuel injectors contaminated; volume supply of fuel pump too low.

Most common problem is an intake air leak. This can explain the high NOx reading. As for the seemingly hot running, be sure the fans both run at low speed. They should come on at 212F and switch to high at 216F. You can use an OBD2 data logger/reader/code reader to view the coolant temperature -- obtained over the OBD2 bus -- in real time.

AOS could be failing. I've had 3 AOS failures and not one intake manifold leak. Only one of the 3 AOS failures my Boxster suffered did the CEL come on. I forget the error codes bu they had me suspecting a bad MAF, but after 2 days of driving and reaching my destination upon starting the engine the next day in the hotel parking lot oil smoke just billowed from the car's exhaust and then I knew what the error codes were trying to tell me.

Up to you if you want to throw an AOS at the behavior. I will mention I went through 3 AOSs with my 2002 and the AOS life was anywhere from around 80K miles to 100K miles. With 90K miles if the AOS is original it is "due".

What you can do on your driveway is with the engine idling that there is no air leak at the oil filler tube cap. Believe it or not my Boxster's cap developed a leak at around 150K miles -- the o-ring shrunk and also looked like it experienced some wear.

Never had this happen to my Boxster but techs told me the intake gasket (o-rings?) can leak.

A smoke test can find an intake leak. A poor man's leak test is to spray some aerosol carb cleaner at where the intake bolts to the heads. If the engine reacts there's the leak.

The spark plug tube o-rings go bad. When the tech did the ones in my car he reported they were just in pieces when he pulled the tubes out. This even though I changed the oil every 5K miles. I suspect over time the o-rings suffer from wear and tear due to the tubes vibrating, even if only a tiny bit. IIRC Porsche changed spark plug tube to do away with the o-rings in later models.

My daughter's '01 also had a bad AOS with no CEL. The symptom on hers was a steady drip - nearly trickle - of oil during operation. No smoke at all.

I've had these codes for a while. I should take the time to resolve it, but...

I did spark plug tubes on the '01 at about 110k. The O-rings were badly shrunk and there was a lot of oil seeping onto the bottom tray. The new ones cost me $35 (I think?) and went in in a couple hours. It took about 25 minutes to do the first one, and about 5 to do the last one.

There was a letter to Panorama 'Tech Q&A' last month where someone asked about these codes. He got what I thought was a rather snippy reply. It annoyed me enough that I wrote a letter to the editor about it.

If those codes have been appearing for a while then it is not likely to be the AOS. My experience is when the AOS goes it goes pretty quick. The 1st one was the most drawn out AOS failure because I stopped driving the car and switched to trying to diagnose the engine symptoms. Had I been able to ignore the initial symptoms and kept driving the car the AOS would have gone downhill in very short order.

The 2nd one in which the CEL would come on towards the end of the day after driving all day -- this on a road trip from CA to the mid west -- and did this for two days. This was the one I thought the MAF was the problem. But the 3rd day when I started the engine after the car sat overnight in a hotel parking lot the billowing smoke from the exhaust clearly indicated the AOS.

The 3rd one was out of the blue. No symptoms at all then pulling away from a stop light I noticed a pretty light colored cloud behind the car. I didn't even think AOS at first but a coolant leak but the dash showed nothing amiss and as I drove on I quickly realized it was oil smoke and that almost certainly the AOS had failed.

The longest time I dealt with a CEL and mixture related error codes pointing to a lean condition at idle/low engine speed was the time what proved to be the oil filler tube cap was leaking. I actually replaced the MAF but the codes came back and I had to look elsewhere. A co-worker heard an air leak when I had the engine idling and the rear trunk lid open and it was the oil filler tube cap. Even though the cap was "tight" the cap no longer sealed well with the tube and the cap had enough play both radially and even a bit axially that I could move the cap about and change the degree of leak from none to some.

The cap leak was only really an issue at low engine speed. This explained why often the CEL only appeared towards the end of my 30 mile work commute drive. The last mile or so would be on surface streets with some idling with the engine fully up to temperature and it was at this time often the CEL would come on. But not every day.

Never had this with my car but some owners have reported the oil filler tube leaks. It cracks or the connection with the block leaks.

I know it's not the AOS. I was thinking intake leak, but the oil filler cap is interesting. Seems like an easy thing to test, too. But I have to look into some creaking and popping in the top mechanism, so I might as well pull the engine cover and blow some cigar smoke into the area while I'm there.