TEST 3: O2 Heater Resistance Test
So far in your testing, you've checked and confirmed that the upstream oxygen (O2) sensor's heater element is being powered with 10 to 12 Volts DC and that the PCM is also supplying the Ground it needs.
The last thing you need to do, to either condemn the O2 sensor as bad (or not), is a resistance test of the oxygen sensor's heater element.
You can check this with the O2 sensor on or off the vehicle. It all depends on if you're able to stick your hands and the multimeter test leads close enough to the O2 sensor's connector to check the resistance of the heater element within it.
OK, this is what you need to do:
- 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!
- Locate the O2 sensor wires that correspond to:
- The circuit C and D (4.8L, 5.3L) or D and E (6.0L).
- 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.
- With your multimeter in Ohms mode, probe the terminals that correspond to the letters C and D (or D and E on the 6.0L) 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). This confirms that the upstream oxygen sensor is bad and needs to be replaced with a new one.
Depending on what upstream 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.
Here's why: You have verified that the upstream 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.
If this info saved the day, buy me a beer!