Upstream O2 Sensor Giving No/Bad reading to ECU

Thyme · Dec 16, 2023

Hey all, I'm having a real headscratcher of a problem with my Bank 2 Upstream O2 sensor. It failed on me out of the blue on a long trip across the country, or so the ECU claims. Its a Bosch O2 sensor, put in the car at the same time as the other functioning upstream O2 (also Bosch).

When the issue is happening, the ECU seems to be reading the sensor voltage as ~0.09-0.11V, or 0V. It'll throw codes for both a failed heater circuit and a failed O2 sensor. I attempted to solve the issue with another O2 sensor, which lead to the same problem. I then replaced the plug on the harness, as I saw the sheathing for the wires near it was damaged. Verified my wiring job and tested continuity on all wires from the sensor to the harness plug near the battery. Heater circuit read 5 ohms on both sensors at both the sensor connector and the harness near the battery, and I was able to see voltage from both sensors when heated with a torch and probed by a multimeter. The only place I haven't checked at is the connector at the ECU itself, as I'm a bit too lazy to dig the thing out at the moment.

The strangest part is this log I took using Forscan (https://files.catbox.moe/cllvrf.fsl). It shows the O2 sensor reading fine, and then suddenly starting to drop off down to the previous voltage range I mentioned before. I've seen this behavior on startup with a cheap OBDII reader viewing live data as well.

Any clues as to what could be screwing up on the car?

Thyme · Dec 16, 2023

Update: Cleaned and reconnected all connectors and took the car for a drive while data logging again (https://files.catbox.moe/m5m87a.fsl). ran fine until ~25 min in, then the O2 sensor failed again. Attached is the screen cap of the failure. I also noticed that the reading of the O2 sensor in question was a lot lower resolution than the Bank 1 O2.

I pulled the O2 connector harness plug when I got home and verified with a multimeter that the voltage reported by the ECU was also the voltage at the harness. This is leading me to believe that my wiring job may be less than satisfactory. I'm going to verify tomorrow morning if the O2 sensor is reading properly or not using the old harness connector.

If the sensor turns out to be good, should I pull the trigger on an entire new harness, or get the proper tools to re-pin the connectors with new wires?

lwarrior1016 · Dec 17, 2023

Looks like that O2 was switching just fine and then went dead. Before jumping to any extremes, you may want to check voltage up closer to the computer before you condemn the wiring. I’ve seen the O2 circuit go bad in the computers.

All that being said, I don’t think you can get a new harness for these any more. You’d likely end up with a used harness and you don’t know what problems that one brings.

Thyme · Dec 17, 2023

lwarrior1016 said:
Looks like that O2 was switching just fine and then went dead. Before jumping to any extremes, you may want to check voltage up closer to the computer before you condemn the wiring. I’ve seen the O2 circuit go bad in the computers.

All that being said, I don’t think you can get a new harness for these any more. You’d likely end up with a used harness and you don’t know what problems that one brings.

ran the car today and verified with a multimeter that it was reporting a similar voltage to the ecu at the backside of the new harness plug I installed. I think it may just be a case of an O2 going bad and getting another bad one as a replacement. Am considering getting an NTK sensor and seeing if the issue gets resolved.

With the O2 circuit in the computer going bad, how would you go about testing that?

lwarrior1016 · Dec 17, 2023

Thyme said:
ran the car today and verified with a multimeter that it was reporting a similar voltage to the ecu at the backside of the new harness plug I installed. I think it may just be a case of an O2 going bad and getting another bad one as a replacement. Am considering getting an NTK sensor and seeing if the issue gets resolved.

With the O2 circuit in the computer going bad, how would you go about testing that?

Definitely get ntk or ford sensors. Bosch are really hit or miss, most times miss.

All I did was test the circuits and verify they were ok on the outside. Checking voltage, and continuity on the wires back to the computer. Replaced O2 sensors twice with the same result, the computer was the last variable. We replaced it and all was good.

Thyme · Dec 19, 2023

So far so good with the NTK sensor. Chalk it up to another case of bad bosch sensors. On the bright side it lead me to fix a potential wiring harness issue before it became a problem.