How to Test the Downstream Oxygen Sensor (2003-2005 GM 5.3L, 6.0L)

Testing the post-catalytic converter oxygen (O2) sensors, to see if they are causing the P0141 or P0161 to light up your check engine light (CEL) on your instrument cluster is not hard to do.

With the testing instructions, in this tutorial, I'll show you how to test them and find out if they are good or BAD.

So, if you have one of the following diagnostic trouble codes:

  1. P0141: HO2S Heater Performance Bank 1 Sensor 2.
  2. P0161: HO2S Heater Performance Bank 2 Sensor 2.

... and your GM 4.8L, 5.3L, or 6.0L pickup (or SUV) is between the years 2003 and 2005, this is the article for you. If this tutorial doesn't cover your specific vehicle, take a look at the index of tutorials here: O2 Sensor Tutorial Index.

Remember, these downstream oxygen (O2) sensors are also known as: post catalytic converter oxygen sensors or Rear oxygen sensors and I'll be using all of these terms throughout this tutorial.

... Before we jump into troubleshooting P0141 and/or P0161 on your GM vehicle, I highly recommend that you take a look at the following oxygen sensor Test primer (if you haven't already):

  1. Testing P0135, P0141, P0155, P0161 O2 Heater Performance Problem.
    1. This article answers a lot of the most common questions like:
      1. Symptoms of a BAD oxygen sensor heater element.
      2. Where are the O2 sensors located?
      3. What tools do I need?
      4. What does the heater inside the oxygen sensor Do?

Circuit Descriptions of
the Downstream Oxygen Sensors

P0141, P0161 -Downstream O2 Sensor Test (2003-2005 GM 5.3L, 6.0L) P0141, P0161 -Downstream O2 Sensor Test (2003-2005 GM 5.3L, 6.0L) P0141, P0161 -Downstream O2 Sensor Test (2003-2005 GM 5.3L, 6.0L) P0141, P0161 -Downstream O2 Sensor Test (2003-2005 GM 5.3L, 6.0L)

I'm sure this isn't news to you... that each post catalytic converter oxygen (O2) sensor has 4 wires in its connector.

Knowing which wires to test (because we can actually test the heater element inside the downstream oxygen sensor) becomes a must to get to the bottom of the P0141 and P0161 trouble codes

Here's what you need to know: Each of the four wires is identified by a letter. These four letters used are: A, B, C, and D.

Of the four wires, we need to worry only about the wire identified by the letter D and letter D. These two are the ones that feed power and ground to the heater element within the downstream oxygen sensor.

Now, in case you didn't know... you'll find these letters embossed on the oxygen sensor connector too. Below, in the charts, you'll find a color description of these wires and their specific job.

NOTE 1: In the chance that the color of the wires below don't match what's on the downstream oxygen sensor on your specific vehicle... the info in this tutorial still applies. Why?... Because you can use the letters (A, B, C, and D) embossed on the connectors to identify the circuits.

NOTE 2: There are two different styles of rear heated oxygen sensors that are used on the GM vehicles covered by this tutorial. To be a bit more specific, the difference is in the oxygen sensor's connectors. The illustrations that I'm using cover both styles of connectors and although the letters (of the circuits) are in different places, the circuit description that each letter represents is the same.

O2 Bank 1 Sensor 2 Circuits (4.8L, 5.3L, 6.0L)
Pin Wire Color Description
A Tan w/ White stripe HO2S Low Signal
A <-> Tan HO2S Low Signal (Canada built)
B Purple w/ White stripe HO2S High Signal
C Black Heater Element Ground (Chassis Ground)
D Pink Heater Element 12 Volts (O2 sensor fuse)
O2 Bank 2 Sensor 1 Circuits (4.8L, 5.3L, 6.0L)
Pin Wire Color Description
A Tan HO2S Low Signal
B Purple HO2S High Signal
C Black Heater Element Ground (Chassis Ground)
D Pink Heater Element 12 Volts (O2 sensor fuse)

TEST INFO: The Basics of
Testing the Oxygen Sensor Heater Element

IMPORTANT: The post catalytic converter oxygen (O2) sensors get extremely HOT with the engine running! Do not test them, per the instructions in this tutorial, if the engine has been running for any amount of time... or you run the risk of severe burns!

If the engine is hot, let it cool down completely. This will give the downstream O2 sensors enough time to cool down too... or you run the risk of severe burns from the exhaust pipe or the downstream O2 sensors themselves!

OK, keeping the above safety precautions in mind.... in a nutshell, the very first tests, to troubleshoot P0141 and/or P0161 is to make sure both Bank 1 Sensor 2 or Bank 2 Sensor 2's heater element is getting both voltage and ground.

With the Key On but Engine Off, the O2 sensor should get 10 to 12 Volts from an oxygen sensor fuse. ground is provided internally by the PCM.

Here are some more specifics:

  1. Power is provided by the D circuit wire.
    1. This wire will be the Pink wire of the O2 sensor connector (engine wiring harness side).
  2. ground is provided by the C wire (4.8L, 5.3L) or the E wire (6.0L).
    1. This is the Black wire of the O2 sensor connector (engine wiring harness side).

If it turns out that the downstream O2 sensor's heater element is missing power or ground, well... you found the causes of the P0141 and/or P0161 trouble codes.

After your multimeter confirms that both power (10 to 12 Volts DC) and ground are present, then the next step, is to measure the resistance (with a multimeter) of the heater element inside the oxygen (O2) sensor to see if it's fried or not.

Alright, I'll stop yakking and we'll get testing DTC's P0141 and/or P0161 in the next few pages are the step-by-step testing instructions you'll need to get to the bottom of the problem.