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99 Boxster with P1124 and P1126

Old 06-24-2019, 04:03 PM
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99 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!
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