TEST 1: Checking For 12 Volts (Pink Wire)

P0135, P0155 -Upstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L) P0135, P0155 -Upstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L)

The oxygen sensor's heater element only gets power when you turn the Key On and/or when the engine is running.

IMPORTANT: Before I go any further, let me tell you that you should never test the O2 Senor's with the engine hot and/or running. Since the oxygen sensor's are bolted to the exhaust manifold or pipe, they get VERY HOT! This could result in severe burns!

With the Key in the RUN position (but Engine OFF), this voltage will range between 10 to 12 Volts DC on your multimeter.

OK, to get our P0135/P0155 DTC troubleshooting under way, this is what you need to do:

  1. Locate the upstream oxygen sensor you need to test
    1. Bank 1 Sensor 1 is on the ‘driver’ side of the engine (P0135).
    2. Bank 2 Sensor 1 is on the ‘passenger’ side of the engine (P0155).
  2. Jack up the vehicle and support it on jack stands (if applicable) to access the oxygen (O2) sensors.
    1. Never trust the jack to hold up the vehicle. Use jack stands!
  3. Disconnect the oxygen sensor.
  4. Locate the Pink wire of the engine wiring harness oxygen sensor connector.
    1. The letter D will be embossed on the connector.
  5. With the Key On, Engine Off, this wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: 10 to 12 Volts are present in the Pink wire with the Key On Engine Off- This confirms that the oxygen (O2) sensor's heater element is getting juice.

The next step is to make sure that the oxygen sensor's heater element is getting a good ground on the C circuit wire. For this test, go to TEST 2.

CASE 2: 10 to 12 Volts ARE NOT present in the D circuit wire with the Key On Engine Off- This usually means that the O2 sensor fuse, in the Engine Compartment Fuse Box is blown.

This also indicates that the other upstream oxygen sensor isn't getting power too. Check the oxygen sensor fuse and if blown, replace it and retest.

TEST 2: Checking Ground (Black Wire)

P0135, P0155 -Upstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L) P0135, P0155 -Upstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L)

In this section, you're gonna' confirm that the oxygen sensor's heater element is getting a good ground.

This ground is provided directly on the chassis of your vehicle and can be easily tested with your multimeter or 12 Volt automotive test light.

You can also do a voltage drop test and you can find this test here: Oxygen Sensor Voltage Drop Test.

This is what you'll need to do:

  1. Disconnect the upstream oxygen sensor (if it isn't already from TEST 1).
  2. Locate the circuit C wire (Black wire).
    1. You'll test the wire that is on the engine wiring harness oxygen sensor connector side.
  3. Place your multimeter in Volts DC mode and:
    1. Connect the red multimeter test lead to battery (+).
    2. Connect the black lead to the Black wire of the engine wiring harness O2 sensor connector.
  4. With the Key On, Engine Off, this wire should have 10 to 12 Volts DC.

Let's take a look at your test results:


CASE 1: The multimeter registered 10 to 12 Volts DC- This test result tells yo that the Pre-catalytic converter oxygen sensor's heater element is getting ground (for this particular oxygen sensor).

The last thing you need to do now, is to check the resistance of the heater element inside the oxygen sensor itself. Depending on the result of this O2 heater element resistance test, you'll know if the O2 sensor is fried or not. For this test, go to TEST 3.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC- Re-check all of your connections and make sure you're testing the correct wire.

If your multimeter still does not indicate the 10 to 12 Volts DC, then the Black wire has an ‘open’ short.

By an ‘open’, I mean that the wire is cut somewhere between it and its ground point. To confirm this, you can use a Jumper Wire to ground this circuit and repeat the test.

Repairing this ground issue will solve the P0135 and/or P0155 issue you're having with your GM pickup, van, or SUV.