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:
- Testing P0135, P0141 O2 Heater Performance Problem.
- This article answers a lot of the most common questions like:
- Symptoms Of A Bad oxygen sensor heater element.
- Where are the O2 sensors located?
- What tools do I need?
- What does the heater inside the oxygen sensor do?
- This article answers a lot of the most common questions like:
- Testing Trouble Code: P0135 (GM 3.8L 1994-1996).
- This tutorial covers testing a P0135 Trouble Code on GM 3.8L 1994-1996 model vehicles.
Circuit Descriptions Of The Upstream Oxygen Sensors
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 | ||
|---|---|---|
| Pin | Wire Color | Description |
| 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) |
Where To Buy The Oxygen Sensor And Save
You could buy the upstream oxygen sensor at your local neighborhood auto parts store but if you call them and price it, it's gonna' cost a lot more than the buying them online.
Check out the link below and compare:
17%TEST INFO: The Basics Of Testing The Oxygen Sensor Heater Element
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 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:
- Power is provided by the D circuit wire.
- This wire will be the Pink one, regardless of the type of O2 sensor GM has installed on your vehicle.
- Ground is provided by the C wire.
- This wire is usually a Black wire.
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 your 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 it's 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.
TEST 1: Checking For 12 Volts (Pink Wire)
NOTE: Before you start testing, just a friendly reminder: Perform all tests with a cold engine to avoid serious burns on your hands and/or fingers.
OK, The very first thing you need to do, is to make sure that the front oxygen sensor on your 3.8L Buick (Chevy, Olds, or Pontiac) is getting power.
You can use 12 Volt Automotive Test Light or a multimeter for this test and TEST 2 but to make sure that you're getting the right amount of Voltage, I recommend using a multimeter. If you don't have one or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing at: easyautodiagnostics.com).
NOTE: This test is done on the O2 sensor's engine wiring harness connector which has female terminals.
OK, to get our P0135 DTC troubleshooting under way, this is what you need to do:
- Locate the upstream oxygen sensor and disconnect it from its electrical connector.
- Locate the Pink wire of the engine wiring harness oxygen sensor connector.
- The letter D will be embossed on the connector.
- With your multimeter in Volts DC mode:
- With the red multimeter test lead, probe the letter D wire of the engine wiring harness connector (this will be the Pink wire).
- Ground the black multimeter test lead.
- With the Key On Engine Off, this wire should have 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: Your multimeter recorded 10 to 12 Volts DC. This is good and let's you know that the O2 sensor fuse is good and supplying power to the upstream O2 sensor.
The next step is to make sure that the oxygen sensor's heater element is getting a good Ground on the C circuit wire. For this test, go to: TEST 2: Checking Ground (Circuit C Wire).
CASE 2: Your multimeter DID NOT record the indicated voltage- This usually means that the O2 sensor fuse is blown.
Check the O2 sensor fuse and replace if necessary and retest.
