1998 2.5L 225,000 miles
It has been running great from all the things I have worked on or replaced in the almost 2 years I have owned it. Last week I got the P0171 lean code. I hooked my OBDII reader and noticed the Bank 1 O2 sensor was reading 0 volts. Never fluctuated at all. I cleared the code just to see if it set it again and 2 days later it came back on. From the list of things could possibly cause this code the O2 sensor is the only option I had. After replacing it, (it looked to be the OEM sensor) I still show zero volts. The STFT % does fluctuate with throttle response. Before and after the replacement I would feel a intermittent rough idle that would last only a few seconds at a time.
My main question is why would I not show any voltage from the O2 sensor?
Before this code I never paid attention to this reading when checking other things. So I'm not sure if the zero Volts has been like this for a long time or just something new. I'm not sure what to check first and I have put about 85 miles on the new O2 sensor and no CEL.

 

O2 sensors need to be above 650degF to work, so are you warming up the engine first?

Computer doesn't use O2 sensor at all at idle or at WOT(wide open throttle) regardless of temp

O2 B1S1 is the sensor you should checking, S2 is the rear O2, B2 is only on V6 or V8 engine

Should see 0.1-0.9v, so no volts could be wiring issue
But I would expect one of these codes
P0130 02 Sensor Circuit Malfunction (Bank I Sensor 1)
P0131 02 Sensor Circuit Low Voltage (Bank I Sensor I)
P0133 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)
P0134 02 Sensor Circuit No Activity Detected (Bank I Sensor 1)

Not just a Lean code, so thats VERY odd

 

O2 B1S1 is the one I'm checking.
Yes it stays Zero Volts 100% of the time. Cold, Hot, doesn't matter. It never changes. And no other codes are set. I will check Key On Engine Off to see if I have 12v to the sensor. The strange thing is after putting in the new O2 sensor the CEL has not come back on.

 

The 12v at the O2 sensor is for the heater inside it, so it warms up faster
The 0.1-0.9volts is generated by the sensor itself once its warmed up

Maybe your reader is not sending/getting correct PID(packet ID), OBD readers send a request to computer for a specific PIDs data

Not that you need to know any of this but PIDs are described here: https://en.wikipedia.org/wiki/OBD-II_PIDs

So the OBD reader sends PID for O2 sensor B1S1 and then gets back that data from computer, if it was sending wrong PID then it would get nothing back

Check if you get voltage for B1S2, the downstream O2 sensor, should be steady at 0.7-0.8v with engine warmed up and running

 

You can never give too much info! I did not know about the PIDs
And no it does not show voltage for sensor 2 bank 1 downstream either. I will try to download a different OBDII app and see what it reads. That makes more sense to me.

 

So I tried 2 other Apps and still does not read the voltage at either O2 sensor. I even tried to enter different years and different vehicles but nothing. I'll probably have to take it to a shop that has a better scanner. On the good side, I've put almost 150 miles on it and still no CEL so maybe the new O2 sensor actually fixed it!

 

Thats odd

You can try a sanity check on the computer, unplug the new O2 Sensor's connector, should get a CEL right away because heater is unplugged but drive until its warmed up, see if it sets another O2 sensor code or two

 

I will definitely try this!

 

I unplugged the new O2 sensor and no CEL. I let it run for about 10 minutes and never got the CEL. I did notice it still had an erratic idle that would come and go. It would last for about 15 seconds at a time. The only thing I can really think is maybe a vacuum leak that I will check for. I also noticed that when I unplugged the IAC the RPM stays the same now when it used to drop way down.

 

Did you get a CEL with IAC Valve unplugged?

Try unplugging MAF sensor

Its VERY odd computer is not seeing O2 unplugged, thats an emissions system sensor, so priority system, lol
US doesn't care if the engine doesn't work, it only cares if it pollutes the air, EVAP, O2s and EGR are emissions sensors

 

No CEL with IAC unplugged either! I had it unplugged for about 1 minute. I can try the MAF sensor.
I did get the CEL on again this morning on my way to work and it's the same P0171 code.