My check engine light is on, and it shows a P0138 code for high voltage, bank 1 sensor 2. I replaced the O2 sensor near the manifold (before the cat) with a Bosch sensor, but the light is still on. Even after I reset the code, it came back. I checked the ground to the engine block, and it’s fine, and my 15A fuses look good too. Do you have any other ideas about why I’m getting a high voltage reading? Could the rear O2 sensor be causing this and need to be replaced? Has anyone else solved this problem?
I had that code too. I replaced the rear O2 sensor, and the code disappeared.
You have the wrong sensor. Sensor 2 is the one after the cat, so you should do more research.
The rear O2 sensor is the one after the cat, as mentioned by X99J above, so replacing the front sensor won’t fix this code.
The P0138 high voltage code means there’s a problem with the rear sensor’s heater circuit. The heater inside the rear O2 sensor might be bad. You can check the heater wires (dark green/white and black) for continuity. If the heater checks out okay, you’ll need to check the rest of the circuit from the power distribution center (PDC) to the ground stud next to the dipstick to find the problem.
When you say to check the circuit and wires, do you mean looking for melted insulation or using a tester? If the wiring looks good, I’ll replace the rear O2 sensor.
I meant using a meter to check the circuit for continuity. But you’re right—looking for melted insulation could also help find the problem. Just be careful when checking the sensor; make sure to test the two heater wires (the colors mentioned above) and not the two sensor output wires. You don’t want to damage the O2 sensor by applying voltage to the output wires, and it won’t help you fix the issue anyway.
What does the 2nd sensor do? Adjust fuel mixture, or is it for emmissions only? I’m getting the P0138 now, it seems to be a fairly common code.
Thanks in advance,
The rear O2 sensor on an OBD-II system checks the oxygen level in the exhaust after it passes through the catalytic converter (cat). This helps compare it with the reading from the front O2 sensor (before the cat) to see if the cat is working properly. The rear sensor doesn’t directly change the air/fuel mix, but it can still have an important indirect effect because both sensors’ heating elements are connected in parallel. If the heater for the rear sensor has a problem and blows a fuse, it will also disable the heater for the front sensor, which can lead to issues with how the vehicle drives.
I’ve got this code as well. Weird.
Yeah, I still haven’t figured it out. I think I’m going to try replacing the rear O2 sensor.
Are the front and rear sensors the same?
Nope, the front sensor adjusts the air/fuel ratio, while the rear sensor only measures the oxygen in the exhaust. Check Mason’s post above.
Front and rear O2 sensors have physically different connectors, otherwise they are the same. That’s why the replacement sensors that require splicing your old connector are the same for front or rear applications.
Where is the fuse located?
My downstream O2 sensor wires (behind the cat) got caught on the driveshaft and wore through. I’m experiencing the same hot engine misfires and sputtering problem mentioned in these posts, along with the same code. After the sputtering started, I spliced the wires together, but the issue is still there. Since I read that the upstream and downstream O2 sensors are on a parallel circuit, I thought I should check the fuse. Do you know where it is? I’m also wondering if the downstream O2 short could have affected the upstream O2 sensor. What do you think?
The O2 sensor heater circuit fuse is in the Power Distribution Center under the hood. If you damaged the wiring to the rear sensor, it probably shorted the heater circuit, which would disable the heater for both sensors.