Did you check for any vacuum leaks? A bad vacuum hose or intake manifold leak will also give you a 4-3-6 code. How old is the front O2 sensor? If you can't find any vacuum leaks try changing the front O2 sensor. The rear O2 sensor is just looking for clean exhaust. Its working right if it's reading zero volts across it's terminals.

Anyway here's what the book says:


Code 4-3-6 (Rear HO2S Compensation)

1) Check for other codes. If Code 4-2-5 is stored, perform
testing for that code first. If Code4-2-5 is not stored, go to next
step.

2) Perform a compression check on all cylinders. Repair as
necessary. If compression is okay, go to next step.

3) Check for air leakage in intake system. Repair as
necessary. If there is no air leakage in intake system, check for air
leakage in exhaust system. Repair as necessary. If there is no air
leakage in exhaust system, check fuel and residual pressure. Repair as
necessary. If fuel and residual pressure are okay, go to next step.

4) Turn ignition on. Connect fuel pressure gauge to system.
Squeeze hose between EVAP valve and intake manifold with Hose Pliers
(115 8957). Code for EVAP system can be set. Start engine and run at
idle until rear HO2S starts operating. Rear HO2S starts operating a
short time after rear HO2S preheating starts.

5) Increase fuel pressure by squeezing hose after fuel
pressure regulator with Hose Pliers (115 8957). Voltage across rear
HO2S terminals should increase to about .8 volt. Remove hose pliers.
Remove instrument cluster hose at intake manifold. If voltage across
rear HO2S terminals drops to about zero volts, retest using new front
HO2S. If voltage across rear HO2S terminals does not drop to about
zero volts, retest using new rear HO2S.






Mike
'95 850T