P0141, P0161 Downstream O2 Sensor Test (1999-2002 GM 4.8L, 5.3L, 6.0L)

TEST 2: Checking Ground (Circuit C Wire)

The post-catalytic converter oxygen sensor's heater element needs Ground to activate. Although it's rare to have a Ground issue, we still need to make sure it's there.

In this section we'll test the Ground circuit with your multimeter in Volts DC mode.

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 (4.8L, 5.3L) circuit wire.
   • You'll test the wire that is on the engine wiring harness connector side.
   • This is the Black of the 4 wires.
3. Place your multimeter in Volts DC mode and:
   • Connect the red multimeter test lead to battery positive (+) terminal.
   • Connect the black lead to the C (4.8L, 5.3L) 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- So far so good, since this test result confirms that the Ground wire is OK.

Now that we know that the post-catalytic converter O2 sensor's heater element is getting voltage and Ground, the next step is to test the heater element, inside the O2 sensor, itself. This is a simple resistance done 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-circuit' problem.

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 P0141 and/or P0161 issue you're having with your GM pickup or SUV.

TEST 3: O2 Heater Resistance Test

In this test section, we're gonna' check the heater element itself (which is inside the O2 sensor).

So far in your testing, you've checked and confirmed:

• The downstream O2 sensor you're testing does have 10 to 12 Volts being fed to it (TEST 1).
• That it also has a good path to Ground (TEST 2).

Depending on what's easier for you, you can check this with the O2 sensor on or off the vehicle.

OK, this is what you need to do:

1. Disconnect the downstream 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 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 the heater element within the oxygen sensor is OK.

CASE 2: Your multimeter showed an open circuit (OL). Replace the downstream oxygen sensor your currently testing, since this result confirms it is bad.

Depending on what downstream oxygen sensor you're testing (either Bank 1 Sensor 2 or Bank 2 Sensor 2), replacing the oxygen sensor will solve the P0141 or P0161 diagnostic trouble code.

Here's why: You know, from your testing, that the O2 sensor's heater element is getting both power and Ground (in TEST 1 and TEST 2 respectively). 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.