Testing P0135, P0141, P0155, P0161 O2 Heater Performance Problem

Troubleshooting DTCs P0135, P0141, P0155, P0161 is not hard and in this tutorial, we'll look at some of the basics you need to know and what tests you need (to solve the oxygen sensor problem these diagnostic trouble codes indicate).

Each one of these diagnostic trouble codes (DTCs) (P0135, P0141, P0155, P0161) indicate a problem with the oxygen sensor's internal heater element and from personal experience I can tell you that diagnosing them is not hard.

Let's get started...

Symptoms Of A BAD Oxygen Sensor Heater Element

The most obvious symptom, you'll see when the O2 sensor's heater element goes BAD, is the check engine light (CEL) shining nice and bright on your instrument cluster.

You'll also see one or several of the following symptoms of a BAD oxygen sensor heater:

  1. Diagnostic trouble codes:
    1. P0135: HO2S Heater Performance Bank 1 Sensor 1.
    2. P0141: HO2S Heater Performance Bank 1 Sensor 2.
    3. P0155: HO2S Heater Performance Bank 2 Sensor 1.
    4. P0161: HO2S Heater Performance Bank 2 Sensor 2.
  2. BAD gas mileage.
  3. Won't pass the emissions test.

How To Locate The Oxygen Sensors

Testing P0135, P0141, P0155, P0161 O2 Heater Performance Problem

Your GM pickup or SUV has a total of 4 oxygen sensors. If you've done any type of research, you've come across terms such as upstream and downstream oxygen sensors.

You've also read that some are located before or after the catalytic converter. You've also noticed that the trouble code descriptions identify the O2 sensors as being on Bank 1 or Bank 2 and then identified as Sensor 1 or Sensor 2.

All this may leave you scratching your head and wondering where exactly these oxygen sensor are.

Bank 1 and Bank 2

To understand the locations of the oxygen sensors, you need to know that the 4.8L, 5.3L or 6.0L engine on your pickup or SUV has two banks of cylinders. One bank of cylinders is known as Bank 1 and the other as Bank 2.

Bank 1 is the one that has cylinders 1, 3, 5, and 7 (located on the driver side). Bank 2 is the one that has cylinders 2, 4, 6, and 8 (this one is the passenger side bank of the engine).

So if you have a P0135 (HO2S Heater Performance Bank 1 Sensor 1) or a P0141 (HO2S Heater Performance Bank 1 Sensor 2)... this means that this oxygen sensor is located on the side of the engine that has cylinders 1, 3, 5, and 7. This bank is the driver side bank of the engine.

So if you have a P0155 (HO2S Heater Performance Bank 2 Sensor 1) or a P0161 (HO2S Heater Performance Bank 2 Sensor 2)... this means that this oxygen sensor is located on the side of the engine that has cylinders 2, 4, 6, and 8. This bank is the passenger side bank of the engine.

Sensor 1 and Sensor 2

Now, to further identify where exactly the oxygen (O2) sensor is on the exhaust pipe, all vehicle manufacturers use the terms Sensor 1 and Sensor 2.

Sensor 1 is always located before the catalytic converter and is usually on the exhaust manifold itself or right near where the exhaust pipe bolts/connects to the exhaust manifold. This O2 sensor is also called the upstream oxygen sensor.

Sensor 2 is always located after the catalytic converter and is also known as the downstream oxygen sensor.

In this tutorial, I'll show you how to check the oxygen (O2) sensor's heater element and the two circuits that feed it with power (10 to 12 Volts) and Ground with a multimeter.

In case you're wondering, where the oxygen (O2) sensor that the code P0135 is accusing of being BAD is... this bad boy is located on the exhaust manifold and before the catalytic converter.

What Tools Do I Need To Test Codes P0135, P0141, P0155, P0161?

Here's a basic list of tools you'll need:

  1. A Multimeter.
    1. You'll only be using the multimeter to check voltage DC and resistance (Ohms).
    2. If you need to upgrade or buy a multimeter, check out my recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing (found at: easyautodiagnostics.com).
  2. Scan Tool.
    1. To actually test the oxygen sensor's heater element, you don't need a scan tool... but it does come in super handy to read the diagnostic trouble code(s).
    2. Don't have one?... check out my recommendation: Actron CP9580 Scan Tool Review.
  3. Wire Piercing Probe.
    1. Although this tool is not an absolute must, if you do buy one, you'll realize just how easy it makes testing the voltages inside the wires.
    2. If you need to see what this tool looks like, you can see it here: Wire Piercing Probe.

What Does The Heater Inside The Oxygen Sensor Do?

Each one of the four oxygen (O2) sensors on your GM pickup, SUV, or van has to reach a certain temperature to activate and start measuring the oxygen content of the exhaust.

As surprising as it may seem, the temperature of the exhaust gas isn't enough to keep the oxygen sensor hot enough to perform 100% of the time.

The solution? Place a heater element inside of it so that it stays hot as long as the PCM commands it.


This is the reason you'll notice that all of the oxygen sensors on your GM pickup, SUV or van have 4 wires.

Two of them are for the heater element inside the oxygen sensor and the other 2 are for the oxygen sensor signal itself. In a nutshell, this is how the heater element activates:

  1. A dedicated oxygen sensor fuse provides the oxygen sensor's heater element with power (battery voltage).
    1. this voltage is provided with the Key in the On Position (RUN position).
    2. So, with the Key On and Engine Off or Engine Running, you'll have power going to the oxygen sensor's heater element.
  2. What activates the heater element to get cooking is ground.
    1. In the earlier 4.8L, 5.3L, and 6.0L... this ground is provided by a wire connected to the chassis of the vehicle.
    2. On later models, this ground is provided internally by the PCM (Powertrain Control Module = Fuel Injection Computer).

If you're here reading this... then you now know that the heater element (inside the oxygen sensor) doesn't last forever (they fail very frequently). Let's turn the page and find out how to test them and where to find the specific O2 sensor tutorial you need...