P0135, P0155 -Upstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L)

TEST 3: O2 Heater Resistance Test

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If you've reached this point, you have already confirmed that the oxygen sensor's heater element is getting both power and Ground.

Power is supplied on the Pink wire and Ground is supplied on the Black wire.

The next step (and the last one) is to check to see if the oxygen sensor's heater element is fried inside the O2 sensor itself by measuring its resistance with your multimeter in Ohms mode.

You can test the O2 sensor's heater element with the O2 sensor on the vehicle or off, whatever is the easiest thing for you to do.

OK, this is what you need to do:

1. Disconnect the 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:
   1. The circuit C.
   2. -AND-
   3. The circuit D.
   4. 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 of the O2 sensor connector.
   • 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 heater element inside the O2 sensor is bad. Replace the oxygen sensor.

Depending on what oxygen sensor you're testing (either Bank 1 Sensor 1 or Bank 2 Sensor 1), replacing the oxygen sensor will solve the P0135 or P0155 diagnostic trouble code.

If this info saved the day, buy me a beer!