Hello Guys,

I need a little help here for a friends car. 1991 Firebird 3.1 V6.
Problem is, its running rich. The exhaust gases are smelly and the inspection shows high carbon monoxide. The cat is defective, again.
Following things were checked:
-Fuel pressure
-Air filter
-logged two sessions with tuner pro

The O2 sensor shows 450mV when warming up from cold, once it gets into closed loop, it slowly pending between high and low, but as soon as you step the gas, it hang around 850mV. Very unlikely, also, the INT is at minimum operating range at 100 and BLM is also pretty low. So its pulling fuel out. Fuel pressure is good! All sensors are working like they should (MAT, TPS, MAP, CTS) except of the O2.

What is causing the O2 Sensor hanging up at 850mV like WOT all the time? The truely sensed rich condition (cause INT 100) OR its faulty and giving wrong values? It has already been replaced 2 years ago like the first OEM cat, which was rattling. I could need some advise here.

Next steps I want to check are:
-ignition timing
-swapping O2 sensor versus a used good one for checking.