I am getting a P0420 code, inefficient cat, I have replaced the o2 sensors (all three - aftermarket brand) - replaced most of the vacuum lines and ran cat-cleaner through the system.

the last thing to do is directly test/replace the cat. I have a hard time believing that the cat is actually bad - but the car does have 160k miles on it, so it is conceivable.

BUT I wanted to exhaust all other options first. I can see the test the ECM fails, which is "Rich to Lean Sensor threshold voltage (constant). Max: 51 Current: 104" which, I assume is triggering the OBD code


Anyone have any thoughts?

 

did you feel around or hear any exhaust leaks noises? yes cats can and do fail. Any notable oil consumption or recent bouts of misfires or poor gas mileage?

 

Thanks everyone, you all have been very helpful. I actually did have the code come back, so I went back to the standard troubleshooting steps:

Easy/Cheap Problems -> Harder/Expensive Problems.

So starting again with the Vacuum Lines, I actually found a leak I had overlooked.

The Vacuum change over switch! I noticed when a little starter fluid was sprayed on it, the engine would react - pretty obvious v. leak. (it was/is sort of tucked under the fuse box so I can see how I missed it the first dozen times)

So, I did a quick fix (glued the **** out of it, ensuring that the brakes were still functional) and ordered a new one. Drove 1200 miles, at various speeds, and the CEL came back. BUUUT with a new code (a VERY good thing) - massive vacuum leak in the evap system. So new gas cap (the most likely culprit) and so far, so good. I'm glad I did all the other fixes, since they were things that needed addressed, and it looks like I'm good to go (passed smog the other day too - so the CAT is fine)

Anyway, hope that helps someone down the line.

-A

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2004 XC90 T6
1997 Mercedes S420