With this tutorial, I'm gonna' help you test the upstream oxygen sensor (also known as the front 02 sensor or the O2 sensor before the catalytic converter) on your 1997-2003 3.8L Chevy (Buick, Olds, or Pontiac) car or mini-van.
This particular tutorial will help you diagnose/test a P0135 Diagnostic Trouble Code, which refers to a Bank 1 Sensor 1 Heater Performance Problem.
This article covers testing the Upstream O2 sensor for the most part and to keep the tutorial short and to the point I've not included a lot of basic O2 sensor info. But don't worry, you can find this basic oxygen sensor test info in this oxygen sensor test primer found here:
If you've already looked at the Pre-Catalytic oxygen sensor on your vehicle... you know that the O2 sensor has 4 wires.
To test the P0135 DTC and effectively troubleshoot the heater element inside the upstream oxygen (O2) sensor... we need to identify what wire does what.
The cool thing, is that besides identifying the wire (we need to test) by its color... we can also identify the wire (circuit) with a letter that GM has embossed on the oxygen sensor's connector.
These letters are always embossed on the O2 sensor connector that belongs to the engine wiring harness (see photo 3 of 3 in the image viewer). Depending on whether the O2 sensor has been replaced or not.... the O2 sensor may have or may not have these letters embossed on it. But the vehicle's O2 sensor connector will always have them.
The two circuits (wires, that is), that we need to concern ourselves with are the ones labeled with the letter C and D and in the chart below, is a short description of what each circuit does and the color of the wire.
NOTE: If the wire color doesn't match those of the Upstream O2 sensor on your specific 3.8L Chevy (Buick, Olds, or Pontiac) vehicle, don't worry, because you can still identify the correct wire (circuit) by the letter embossed on the oxygen sensor connector and the testing info in this tutorial will still apply.
|O2 Bank 1 Sensor 1 Circuits|
|A||Tan||HO2S Low Signal|
|B||Purple w/ White stripe||HO2S High Signal|
|C||Black w/ White stripe||Heater Element Ground (Provide by PCM)|
|D||Pink||Heater Element 12 Volts (O2 sensor fuse)|
The very first thing we need to do, is to verify that the upstream oxygen sensor's heater element is getting voltage.
You'll test this voltage with the Key On, but Engine Off. The amount of voltage your multimeter should record will be between 10 to 12 Volts DC.
The next step, after confirming that Voltage is present, is to make sure that Ground is also present.
Here are some more specifics:
Making sure that the upstream oxygen sensor is getting both power and ground are the two first basic tests we need to do.
Now, if both Power (10 to 12 Volts DC) and Ground are present, then the next step (and the last test), is to measure the resistance of the heater element inside the oxygen (O2) sensor to see if it's fried or not.
This last resistance test you'll do with you multimeter in Ohms (Ω) mode and, depending on the result, you'll know if the heater element (within the Upstream O2 sensor) is fried or not.
As you can see, testing the O2 sensor before the catalytic converter, to see if its BAD or not, is not hard to do at all.... OK, in the next few pages are the step-by-step testing instructions you'll need to get to the bottom of the problem.
“Remember, this hobby is supposed to be your therapy, not the reason you need therapy.”