DTC P0135 – How To Service Your 02 Sensors

DTC P0135 – How To Service Your 02 Sensors


The P0135 Check engine light is a OBDII Trouble code informing you that there is a problem with one of your oxygen sensors. Your vehicle’s ECU or PCM relies on the signal from the oxygen sensors to operate your engine for closed loop control. Unfortunately if you have the P0135 OBDII trouble code, one of your 02 sensors is not getting the proper voltage from the heater wire.

This Trouble Code can mean that the Upstream Oxygen Sensor has failed and that the Heater Element has gone out.

There can often be other trouble codes that accompany this one, from the DTC P0141, DTC P0135,  DTC P0138 or DTC P0420. But today we’ll be addressing this issue in a 2001 Honda Accord ULEV 4 cylinder. We will be testing the wiring in your upstream 02 sensor before replacing it and showing you what steps to take along the way.



Before working on your vehicle, disconnect the negative terminal on your battery. Make sure to locate your radio code if you are still rocking the factory stereo.

After disconnecting your battery, locate your primary upstream 02 sensor. This is the first step in our How To Service Your 02 Sensors guide. Your upstream 02 sensor is located in the header b pipe and can be easily accessed from the front of the motor.



Follow the 02 sensor and locate the 02 sensor connector that is mounted to the front of the transmission mount. This clip is held on with a 10mm bolt, which you can remove now or simply unplug the clip.



To undo this connector and slide the 02 sensor off, find the backside of the plug and depress the backside of the clip.


Push down on the circled area and now the front upstream 02 sensor harness can now be pulled off the mounting tab.


Next push down on the connector locking mechanism and unplug the front upstream 02 sensor. With the 02 sensor unplugged, you can now see the wiring of the connector as shown below.

With your upstream 02 sensor now unplugged it’s time to get down to the actual how to service your 02 sensors checklist. As the diagram above illustrates, there are differences in color depending on what year Accord you own.

The wiring for the 1994-1997 Accord is Yellow with a black stripe, while the 1998-2002 Honda Accord year ranges will be black with yellow stripe. Now using your multimeter or similar testing device, you will be testing the voltage at the heater wires to confirm voltage.


Turn the vehicle to the ON position, but do not start the engine. Turn your multimeter to DC Volts and ground the negative terminal on your voltmeter. Now probe the Heater Element Power wire on your 02 sensor connector, this should be giving you 10-12 volts of power.

If you have power at this wire under these conditions, you have confirmed that the upstream Oxygen Sensor’s Heater Element is getting power. If you DO NOT have power to this wire, then you know that the reason the P0135 Trouble Code is lighting up the Check Engine on your Honda Accord is a lack of power to the heating element. Your wiring, your PCM or ECU or your 02 sensor are at fault. This is the first step in our how to service your 02 sensors walkthrough.

Next you will be checking the Heater Element Ground wire which is orange with black stripe for the 1994-1997 Honda Accord, and black with yellow stripe for the 1998-2002 Honda Accord.


To test the ground wire on your upstream 02 sensor’s heater element, connect the positive terminal on your multimeter and then put the ground terminal to your heater element ground. This should complete the circuit, proving that your upstream 02 sensor does indeed have the voltage required for operation.

If you do not have a good ground here, you must fix this part of your harness if you expect your trouble code to go away. If you do have a good ground, the next step is to test the heater element portion of your upstream 02 sensor.

Take your multimeter and now we’ll be testing the resistance of the 02 sensor by putting the multimeter into ohm mode. Put both of your meter leads to the heater element ground and power. The resistance range you are looking for is between 10 – 40 Ω on the third and last test of our how to service your 02 sensors guide.

If no resistance value is returned, or you find an Overlimit (OL) value being presented to you by the multimeter, you have a faulty 02 sensor that must be replaced.

Some times you may find your 02 sensor has both voltage and grounds and checks out okay. Because this check engine code can also arise from improper closed loop operation, try disconnecting your battery or clearing your codes to see if the code returns. Closed loop control can be read about in our how to tune closed loop article here.

Congratulations you have now completed our how to service your 02 sensors article for our Honda Accord and cleared your DTC P0135 check engine light.

Have any questions or problems? Leave us a comment below!