P0135 Error Code

[pantone149]

I've got a 99 Subaru 2.5 installed in a 73 VW bus. I'm using the SmallCar interface board to eliminate a bunch of OBD2 errors, but can't get rid of a P0135 error (O2 sensor heater). All tests are positive in that I am getting 12 volts to the sensor heater circuit and the sensor is good (it's new and gives me about 4.5 ohms across the white wires). I've run continuity tests on all the sensor wires and all are good. Will this error code matter with engine performance? can I eliminate with FreeSSM? Should I just ignore the code? That's what I have been told but if having that code changes performance and fuel economy I want it gone.

[jhoefer]

P0135 is the front O2 sensor (pre-cat, wideband) heater. If the ECU thinks the heater isn't working, it may take longer to switch into closed loop mode which will harm fuel economy. The ECU can't trust the sensor readings if its not up to temperature.

1) Disconnect O2 sensor from harness.

2) Heater positive wire goes to switched +12V. On the harness side of plug (not sensor side), place red(+) multimeter lead on the heater positive wire, place black(-) multimeter lead on ground (or battery negative). With key on, you should read 12V. If not, check fuse and wiring.

3) Heater negative wire goes to ECU. On the harness side of plug, place black(-) multimeter lead on the heater negative wire. Place red(+) multimeter lead on battery positive (or other +12V source). With key on, you should read 10-12V. If not, ECU is not providing a ground to the heater circuit. Check wiring or ECU fault (rare).

4) Measure resistance across heater element on sensor side of plug. Should be around 4.5 Ohms, which you've already verified.

The ECU measures the O2 sensor heater directly, is the interface board sitting between the ECU and O2 sensor for some reason?

[pantone149]

1) Disconnect O2 sensor from harness = This test completed pass.

2) Heater positive wire goes to switched +12V. On the harness side of plug (not sensor side), place red(+) multimeter lead on the heater positive wire, place black(-) multimeter lead on ground (or battery negative). With key on, you should read 12V. If not, check fuse and wiring. = This test completed: pass.

3) Heater negative wire goes to ECU. On the harness ide of plug, place black(-) multimeter lead on the heater negative wire. Place red(+) multimeter lead on battery positive (or other +12V source). With key on, you should read 10-12V. If not, ECU is not providing a ground to the heater circuit. Check wiring or ECU fault (rare). Have not done this test, thanks. I'll give it a go tonight.Assuming this is the problem is it OK to take the heater negative wire and attach it to body ground and let the body provide the ground that the ECU is failing to provide?

4) Measure resistance across heater element on sensor side of plug. Should be around 4.5 Ohms, which you've already verified. Tested and pass

The ECU measures the O2 sensor heater directly, is the interface board sitting between the ECU and O2 sensor for some reason? Should not be sitting between ECU and O2.

[pantone149]

Actually for test # 2 I tested with voltmeter red probe connected to positive on the harness side and black probe to body ground and got 12 volts. Then I tested with red probe to positive on the harness side and black probe to the other heater terminal on the harness side and also got 12 volts. This told me that I have 12 volts going into the sensor heater side.

[jhoefer]

Then I tested with red probe to positive on the harness side and black probe to the other heater terminal on the harness side and also got 12 volts. This told me that I have 12 volts going into the sensor heater side.

That's fine. The reason for testing them separate is in case one side has failed, you can still verify the other side.

Since the O2 sensor heater and wiring seems to check out, about the only other suggestions would be to unplug the ECU and run #3 check again to verify you get no reading (the wire isn't shorted to ground somewhere), plug everything back together and clear the code and see if it comes back. Ideally, you'd try a different sensor too, but not sure what access you have to salvage yards to grab one.

Assuming this is the problem is it OK to take the heater negative wire and attach it to body ground and let the body provide the ground that the ECU is failing to provide?

No, that would make the heater work, but the ECU wouldn't know about it which is the problem in the first place.

[pantone149]

I think I learned the problem. The ECU is not grounded. It's simply screwed to wood and insulated from the body.

[pantone149]

Oops. Wrong assumption. Grounding the ECU to body did nothing. Still get the P0135. I do this test after removing the positive lead to the battery overnight, just to make sure all codes are cleared before starting any tests again. Also noticed that the heater sensor resistance is about 3.5 ohms and not 4.5 ohms. Not sure if that matters or not. I have another Bosch sensor so I'll try it and see if anything changes.

[jhoefer]

OBDII codes don't clear from disconnecting the battery as they are stored in non-volatile memory. Use a scan tool to do it if you can't do the multiple drive cycles it takes for the ECU to clear itself (assuming there is no fault anymore).

[pantone149]

Does the upstream (#1) sensor have three wires or four wires? I've been dealing with a four wire sensor and four wire connector. Also, I've been also clearing the codes with a hand-held OBD2 reader.

[itawolf]

Also are you running a 4 wire universal o2-- I've seen a lot of probs with them
you can try a oem
To check for a short in the harness: 

1) With the ignition off unplug the ECM and the front O2 sensor. 
2) Place the black meter lead on chassis ground.
3) Place the red meter lead on one of the wires for the O2 sensor connector.
4) Take a resistance reading (Ohms) You should get 0.L (infinite). 
5) Repeat steps 2-4 for all wires in the O2 sensor connector. 

If you get anything but O.L there is a short to power/ground on that particular wire. You could also do this test on the ECM connector but I wouldn't recommend it since those pins are very small and more fragile. 

[pantone149]

P0135 is an error from the #1 sensor heater. I'm getting the suspicion that I should be using a three-wire sensor, connected to the three-wire (triangular) connector, as my #1/upstream sensor and letting the SmallCar interface board eliminate any errors coming from the four wire/downstream (square connector) sensor. Right now I am using a four-wire sensor connected to a four wire connector and I have a feeling this is the downstream sensor wiring that I really don't need to have. I wonder if the four wire sensor is working perfectly, which explains zero errors from the #2 sensor, and if I need to find the three wires that normally go to the three-wire/#1 sensor and connect to an upstream sensor.