TEST 1: Checking For 12 Volts (Pink Wire)

Before you start, I just want to remind you that the upstream oxygen (O2) sensor needs to be cold. So if the engine has been running for any amount of time, then let the engine cool down for about 45 minutes (or you run the risk of severe burns).

OK, the very first thing you need to do, is to see if the upstream oxygen sensor's heater element is getting voltage. This Voltage will be in the range of 10 to 12 Volts DC with the Key On Engine Off.

This voltage is what powers the heater element (within the O2 sensor) and if this power is missing, then the O2 sensor's heater won't work.

You can use 12 Volt automotive test light or a multimeter for this test and TEST 2 but to make sure that you're getting the right amount of Voltage, I recommend using a multimeter (don't have one or need to upgrade yours? Check out my multimeter recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing at: easyautodiagnostics.com).

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: Your multimeter recorded 10 to 12 Volts DC. This is the normal and expected test result and tells you that the O2 sensor heater is getting power.

The next step is to make sure that the oxygen sensor's heater element is getting a good Ground on the C (4.8L, 5.3L) or E (6.0L) circuit wire. For this test, go to: TEST 2.

CASE 2: Your multimeter DID NOT record the indicated voltage- 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 (Circuit C Wire)

So far, in TEST 1, you confirmed that the O2 sensor's heater element is getting power.

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

This Ground is provided directly by the PCM (Powertrain Control Module = Fuel Injection Computer) and you must take care not to short this wire to 12 Volts or you'll fry the PCM.

This is what you'll need to do:

  1. Disconnect the front (upstream) oxygen sensor (if it isn't already from TEST 1).
  2. Locate the circuit C (4.8L, 5.3L) or E (6.0L) wire.
    1. You'll test the wire that is on the engine wiring harness connector side.
    2. This wire will be one of several colors, depending on the type of O2 sensor.
    Consult the upstream O2 sensor Connector Pin Out Charts for more info: Circuit Descriptions Of The Upstream Oxygen Sensors.
  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 C (4.8L, 5.3L) or E (6.0L) circuit wire of the engine wiring harness connector side.
  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- So far so good, since this test result confirms that the PCM is Grounding the oxygen sensor's heater (for the particular O2 sensor your testing).

The next test, and the last one, is to see if the heater element itself is bad or not. This is a simple test that requires you to check the heater elements resistance with your multimeter. 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-circuit’ problem.

By an ‘open’, I mean that the wire is cut somewhere between it and the PCM connector or that the PCM is bad (although a bad PCM is rare).

Repairing this Ground issue will solve the P0135 and/or P0155 issue you're having with your 2003-2005 GM pickup, van, or SUV.


Cadillac Vehicles:
  • Escalade 5.3L
    • 2006, 2007, 2008, 2009, 2010
Chevrolet Vehicles:
  • Avalanche 5.3L
    • 2006, 2007, 2008, 2009, 2010
  • Silverado 1500, 2500, 3500 4.8L, 5.3L, 6.0L
    • 2006, 2007, 2008, 2009, 2010
  • Suburban 1500, 2500 5.3L, 6.0L
    • 2006, 2007, 2008, 2009, 2010
Chevrolet Vehicles:
  • Tahoe 5.3L
    • 2006, 2007
GMC Vehicles:
  • Sierra 1500, 2500, 3500 4.8L, 5.3L, 6.0L
    • 2006, 2007, 2008, 2009, 2010
  • Yukon Denali 5.3L, 6.0L
    • 2006, 2007, 2008, 2009, 2010