Testing Trouble Code: P0135 (GM 3.8L 1997-2003)

TEST 2: Checking Ground (Circuit C Wire)

As mentioned earlier in this tutorial, the heater element inside the oxygen sensor needs Ground to activate.

In this test section, you'll check this Ground much the same way you tested for power.

NOTE: This test is done on the O2 sensor's engine wiring harness connector which has female terminals.

This is what you'll need to do:

1. Disconnect the rear oxygen sensor (if it isn't already from TEST 1).
2. Locate the circuit C.
   • You'll test the wire that is on the engine wiring harness connector side.
3. Place your multimeter in Volts DC mode and:
   • Connect the red multimeter test lead to battery (+).
   • Connect the Black lead to the C circuit wire of the engine wiring harness connector side.
4. 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: The multimeter registered 10 to 12 Volts DC- Good deal, with this test result, we now know that the O2 sensor is getting Ground.

Now that we have confirmed that the O2 sensor connector is feeding both Power and Ground, the next and final test, is to see if the heater element itself is bad or not. This is a simple test that requires you to check the heater element's resistance with your multimeter. For this test, go to: TEST 3: O2 Heater Resistance Test.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Re-check all of your connections and make sure you're testing the correct wire.

If your multimeter still does not indicate the 10 to 12 Volts DC, then the Black wire has an ‘open’.

By an ‘open’, I mean that the wire is cut somewhere between it and its Ground point. To confirm this, you can use a Jumper Wire to Ground this circuit and repeat the test.

Repairing this Ground issue will solve the P0135 issue you're having with your 3.8L Buick (Chevy, Olds, or Pontiac) vehicle.

TEST 3: O2 Heater Resistance Test

The last thing you now need to do, is to check the O2 sensor's heater element itself with your multimeter in Ohms (Ω) mode.

This resistance test will tell you if the heater element is fried or not, and if it's fried, then you can now buy a new oxygen sensor knowing it's gonna' solve the problem.

NOTE: This test is done on the O2 sensor's connector and not on the engine wiring harness connector. The O2 sensor's connector (which we need to test in this test section) has male spade terminals.

OK, this is what you need to do:

1. Disconnect the upstream oxygen sensor from the engine wiring harness connector (if it isn't already from the previous tests).
   • NOTE: The O2 sensor must be disconnected from the vehicle's connector for this test!
2. Locate the O2 sensor wires that correspond to:
   • The circuit C and D.
   • Both of these letters should be embossed on the O2 sensor's connector to aid you in further identifying the circuits you need to test.
3. With your multimeter in Ohms mode, probe the terminals that correspond to the letters C and D.
   • NOTE: Remember, you're testing the oxygen sensor itself.
   • If all is OK, you should see about 5 to 16 Ω (Ohms).
   • If the heater element is fried, your multimeter will show an open (usually indicated by the letters OL).

Let's take a look at your test results:

CASE 1: Your multimeter showed the indicated resistance. This tells you that you that the heater element within the oxygen sensor is OK.

CASE 2: Your multimeter showed an open circuit (OL). This confirms that the Upstream (Bank 1 Sensor 1) O2 sensor's heater element is fried.

Replacing the oxygen sensor with a new one will solve the P0135 Diagnostic Trouble Code.

Here's why: You have verified that the upstream O2 sensor's heater element is getting 10 to 12 Volts DC in TEST 1 and that it's also getting Ground in TEST 2. In this test section, you have confirmed that the heater element is fried (since it shows an OL on your multimeter), therefore you can conclude with confidence that the O2 sensor is bad.