1990 LT5 emissions fail

[scottfab]

Originally Posted by batchman

Am amazed, and then again not, that a dealer would prefer to shotgun $4k of the wrong parts rather than diagnose your actual problem.

Easy to see why. They probably have a senior tech that is 20 something right out of WyoTek and is only given 30min to troubleshoot.

Originally Posted by batchman

Cats only catalyze HCs..

Cats do indeed convert NOX also (to nitrogen and oxygen)
However, you're correct that with an upstream O2 sensor the cat is not likely the problem.

Originally Posted by batchman

While failing cats can lead to mixture problems that can show up at NOXs that's really only on an OBDII car where the after-cat sensor is getting out of range. I've not looked specifically at this recently enough to recall but I'm pretty sure these cars only have an O2 sensor before the cat so a failing cat that is not blocked won't have an effect on mixture, or anything other than HCs for that matter.

Glad you've got it running better and hope that clears your issue!
- Jeff