Hello m8s, so my bank 2 sensor 1 (passenger side upstream O2) is pegged at 0.9. Vehicle is 2003 D2 with 155k miles

I first tried to clean the connection by disconnecting it and spraying MAF cleaner in there, it was pretty oily. No change. So I changed the O2 sensor with a new bosch unit (I know walker is best, but I had it laying around), once again - no change

What does being stuck at 0.9 mean - bad ground? heater circuit not working? I understand it's a rich condition, what can I check next?

Thanks

 

Get on top of engine and check the wiring very carefully. Specially the wires that go into the plug.

 

The only way that happens is you actually have a rich condition on that bank. They always fail at zero volts, not .9. You can swap 02's left and right and see if it follows the o2, but it probably won't. What are you using to measure it? Best thing is the free torque app with an ELM327 OBDII reader so you can see the graphs. Check it on stone cold startup , and then at or near WOT. If you have a leaking injector it likely isn't leaking enough to be rich at WOT or on cold startup. What does the other side read? Is is cycling? Still rich even at WOT or cold start? Pull some vacuum lines off the intake until it goes lean. I would recommend in order from small to large - SAI feed port, brake booster port, breather port, IACV port.

 

Very interesting input, I'll try pulling vacuum lines. The other O2 is cycling correctly and the short term fuel trim is appropriate. I tested the O2 disconnected with a multimeter, it was 0.5V not cycling, though it wasn't plugged in so the heater circuit wasn't running. After I plugged in the O2 I read the value (with torque app), it slowly increased from 0.5V to 0.9V as it warms up. I'll check it at WOT and get back to you.

The O2 sensor was all black when I pulled it out - may really be that rich. I Guess I could pull spark plugs and see if one is burning richer

 

I was planning on pulling vacuum lines seeing if I could induce a lean condition, however now the O2 sensor is stuck at 0.0-0.1V (per torque). I verified it with a multi meter. Put my hand over the intake to induce rich condition but that O2 doesn't budge.

I've ordered a new walker O2 - surprisingly only $26 on rockauto

Secretly I was hoping one of the injectors was leaking so I could swap them all out

 

Just to add some closure to the thread, changed the O2 sensor (from bosch to walker), everything is right again in the world. I'm guessing the Bosch sensor was originally pegged at 0.9V before it failed at 0V

 

You will find out very shortly if it was in fact a bad o2 or something else