Testing Diagnostic Trouble Codes: P0135, P0141, P0155, P0161

This article will help you to diagnose diagnostic trouble codes: P0135, P0141, P0155, P0161 on your GM 2.2L, 2.4L, 3.1L, 3.4L, 3.8L, 4.3L, 5.0L, 5.7L. All of these trouble codes have to do with a malfunction in the oxygen sensor's heater or it's circuit.

For the GM 4.8L, 5.3L, and 6.0L Oxygen Sensor Heater Performance Diagnostics, go here: Testing P0135, P0141, P0155, P0161 O2 Heater Performance Problem (GM 4.8L, 5.3L, 6.0L).

This article is divided into several headings and they are:

  1. Important Suggestions and Tips
  2. Symptoms of a BAD Oxygen Sensor
  3. Oxygen Sensor Trouble Code Basics
  4. O2 Sensor Heater Tests
  5. Where are the O2 Sensors Located?
  6. Where To Buy the O2 Sensors and Save $$$
  7. More Test Articles

Important Suggestions and Tips

TIP 1: An oxygen sensor gets super hot, since they're attached to the exhaust pipe, and can cause severe burns (especially if the engine has been running). Be careful, use common sense and think safety all of the time.

TIP 2: This article covers most of the GM vehicles with a four wire oxygen sensor. Whether they're upstream or downstream O2 sensors, this info applies to them all.

In case you're wondering why I'm lumping them all together in one article, it's because they all have an internal heater and behave in the exact same way, which means they can all be tested in the same way too.

TIP 3: This article will only help you with an O2 sensor heater code. If you need to see the full oxygen sensor test, you can find them here:

  1. 2.4L Oxygen Sensor Test.
  2. 3.1L, 3.4L Oxygen Sensor Test.
  3. How to Test the Upstream Oxygen Sensor P0131, P0132, P0133 (GM 3.8L).
  4. 4.3L, 5.0L, 5.7L Oxygen Sensor Test.

TIP 4: Some GM 4 cylinder engines (like the early 2.2L) use a single wire upstream O2 sensor that does not have an internal heater. In this application, only the downstream O2 sensor has an internal heater.

Symptoms of a BAD Oxygen Sensor

When the O2 sensor's heater fails, you usually don't even know it. As a matter of fact, usually the only hard symptom is the check engine light (CEL) shining nice and bright to let you know there are some codes stored in the PCM's memory.

Don't get me wrong, a failed oxygen sensor does cause trouble. The two places that a failed O2 sensor has a direct impact on is the environment and your wallet. The environment because now your vehicle is polluting more and in the wallet because your gas mileage will suffer.

There are other symptoms, of course, and these symptoms are:

  1. One or more of the following diagnostic trouble codes will stored in the PCM's memory:
    1. P0135 Heated Oxygen Sensor (HO2S11) Heater Circuit Bank 1 Sensor 1.
    2. P0141 Heated Oxygen Sensor (HO2S11) Heater Circuit Bank 1 Sensor 2.
    3. P0155 Heated Oxygen Sensor (HO2S11) Heater Circuit Bank 2 Sensor 1.
    4. P0161 Heated Oxygen Sensor (HO2S11) Heater Circuit Bank 2 Sensor 2.
  2. Really BAD gas mileage.
  3. Won't pass state mandated emission testing.

Oxygen Sensor Trouble Code Basics

The common thread, that runs through all of the trouble code descriptions for P0135, P0141, P0155, and P0161 is a heater circuit malfunction.

Why would anyone want to put a heater in an oxygen sensor? The short answer is to make sure that the oxygen sensor activates ASAP and stays activated:

In a nutshell, the oxygen sensor needs a specific amount of heat to activate and produce a signal. If the oxygen sensor does not reach this temperature threshold, it does not produce a signal the PCM can use to start regulating the air/fuel mixture.

You would think that since the O2 sensor is placed in the hot exhaust gas stream, this would be enough to activate the oxygen sensor(s) as soon as the engine is started, but this is not the case. Most of the time, the exhaust is not hot enough to maintain the O2 sensors at their ideal temperature threshold to stay activated.

So, automotive engineers came up with a brilliant solution: installing a heater in the oxygen sensor to make sure it's always in the correct temperature zone to activate and stay activated (as long as the engine is running).

The heater inside the oxygen sensor is controlled by two of the four wires that come out of it. One provides 12 Volts, and the other is the ground circuit that the PCM provides internally. So, when the heater is functioning correctly, these 12 Volts and ground are what get it to heat up.

And so, when you see DTCs P0135, P0141, P0155, and P0161 lighting up the check engine light (CEL), its because there's a problem with the heater inside the O2 sensor or its circuit.

Around 98% of the time, you can assume correctly (without any specific testing) that a new O2 sensor is in order and will solve the problem. In the other 2% of the cases, the oxygen sensor heater fuse has blown because the O2 sensor's wires have shorted and melted together (usually because they've come into contact with the exhaust pipe).

Let's turn the page and I'll tell you the two things you need to look for...