If your check engine light is on and you have a diagnostic trouble code P0135 stored in the PCM's memory... then you've come to the right place to test and troubleshoot the upstream oxygen sensor.
A P0135 DTC says: HO2S Heater Performance Bank 1 Sensor 1... and this means that the PCM (Powertrain Control Module = Fuel Injection Computer) has detected a problem with the front oxygen sensor's heater element or a problem in the heater's circuit (wires).
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.
To help you navigate this article, here are its main points:
- Symptoms Of A BAD Oxygen Sensor Heater Element.
- What Tools Do I Need to Test Code P0135?
- What Does The Heater Inside The Oxygen Sensor Do?
- Circuit Descriptions Of The Downstream Oxygen Sensor.
- TEST 1: Making Sure The O2 Heater Is Getting Power.
- TEST 2: Making Sure The O2 Heater Is Getting Ground.
- TEST 3: Testing The Heater Element's Resistance.
You can find this tutorial in Spanish here: Cómo Probar El Código P0135 Con Multímetro (3.5L GM) (at: autotecnico-online.com).
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.
Not only that... but no matter how times you erase the P0135 diagnostic trouble code (DTC), it'll come back as soon as your start and let the engine idle or on the first road test after clearing the DTC.
You'll also see one or several of the following symptoms of a BAD oxygen sensor heater:
- Diagnostic Trouble Codes:
- P0135: HO2S Heater Performance Bank 1 Sensor 1.
- BAD gas mileage.
- Won't pass the emissions test.
What Tools Do I Need To Test Code P0135?
Here's a basic list of tools you'll need:
- A multimeter.
- You'll only be using the multimeter to check voltage DC and resistance (Ohms).
- 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).
- Scan Tool.
- Don't have one?... check out my recommendation: Abe's Scan Tool Recommendation.
- Wire Piercing Probe.
- 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.
- 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?
The oxygen (O2) sensor 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.
The only thing that sucks about this... is that the heater element (inside the oxygen sensor) doesn't last forever and fails very frequently.
Circuit Descriptions Of The Downstream Oxygen Sensor
You'll notice that the Bank 1 Sensor 1 oxygen sensor has 4 wires coming from it (or going to it -depending on if the glass is half full or half empty).
These four wires have a very specific job to do. The two that we're gonna' be concerned with are the one for circuits D and E.
To get into more specifics, if you take a close look at the oxygen sensor connectors (engine wiring harness connector and the O2 sensor connector), you'll see that four letters are embossed on both of them.
These four letters are A, B, C, D and the letter E. These are the letters that you'll need to test the rear oxygen sensor with the instructions in this tutorial.
IMPORTANT: The color of the wires below are probably not gonna be the ones on your particular vehicle... and this is OK. Why? Because the circuit descriptions are all the same irrespective of the color of the wire.
You can still identify the wires to be tested with the letters embossed on the O2 sensor connector.
|Heated Oxygen Sensor Circuits|
|A||Tan w/ White stripe||HO2S Low Signal|
|B||Dark Green||HO2S High Signal|
|D||Pink||Heater Element Battery Voltage|
|E||Dark Green||Heater Element Ground Circuit|