I had a prior thread, but figured I would make this it's own thread. I've been having terrible gas mileage, and i'm trying to track down the problem. I rented a diagnostic scanner from VatoZone, ran the complete test, O2 sensors came back ok, but the catalytic converter test came back "Inc". Messing with the Jeep yesterday, and removed the downstream O2 sensor from the exhaust (left it plugged in, though). Drove around with it just hanging there for about 10-15 minutes (got it up to operating temp) and the CEL never came on. My question is, removing the downstream O2 sensor from the exhaust should trip the CEL, correct?

 

No, you just tricked the PCM into thinking the cat convertor is working. The upstream O2 should switch between rich and lean pretty fast and steady, but the downstream O2 will stay pretty flat if the cat convertor is functioning.

 

What if I unplug and remove it completely, that should trip it, right?

 

Keep it plugged in to the harnes.

 

I'm not going to leave it unplugged, it's just a test. I'm not sure if there's a problem with my OBD, and is supposed to be throwing a CEL but isn't. Something's not right, i'm just trying to figure out what it is.

 

Not plugged in = CEL, plugged in but not screwed in = no CEL.

 

Got it...thanks!