The 1994-1995 Nissan 3.0L Pathfinder (and Pick Up) uses a single oxygen sensor that has an internal heater.
When this heater fails, the oxygen sensor doesn't start working fast enough or doesn't stay working consistent manner.
You don't need any expensive diagnostic test equipment. All you'll need is a multimeter. I'll show you the 3 basic tests you'll need to perform to find out.
Contents of this tutorial:
You can find this tutorial in Spanish here: Calentador Del Sensor De Oxígeno (1994-1995 3.0L Pathfinder) (en: autotecnico-online.com).
Circuit Descriptions Of The 3-Wire Oxygen Sensor
The oxygen sensor needs to heat up to about 600°F to operate and sense the oxygen content of the exhaust stream it's exposed to.
Unfortunately, the engine's exhaust cannot warm up the O2 sensor fast enough nor maintain it hot enough on a consistent basis. So, the only way to get the O2 sensor to reach it's operating temperature (within seconds of the engine starting) and keep it operating is with an internal heater.
The cool thing is that you and I can test the O2 sensor's heater element to see if it's fried or not. To test the oxygen sensor's heater, we need to know what wires to test.
Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector for the heated oxygen sensor:
|3-Wire Oxygen Sensor Pinout
(1994-1995 3.0L Pathfinder and Pick-up)
|A||BLK/YEL||Heater Ground (-)|
|C||BLK/WHT||Heater Power (+)|
TEST 1: Verifying The Heater Element Is Getting Power
To get our heated oxygen sensor diagnostic under way, the first thing we'll do is check that the black with white stripe (BLK/WHT) wire, of the engine wiring harness oxygen sensor connector, is feeding the heater 12 Volts DC.
Checking for these 12 Volts is easily (and accurately) done with a simple multimeter in it's Volts DC mode.
CAUTION: The oxygen sensor gets and stays very hot even after the engine is off! Perform this test with a completely cold engine. Be careful and take all necessary safety precautions! If you raise your vehicle with a jack, place it on jack stands!
IMPORTANT: The pinout in the illustration above is of the connector on the oxygen sensor itself. To check for power, you need to test the BLK/WHT wire of the engine wiring harness sensor connector.
OK, this is what you'll need to do:
Locate the heated oxygen sensor and disconnect it from its harness connector.
Locate the BLK/WHT wire of the engine wiring harness oxygen sensor connector.
With your multimeter in Volts DC mode, probe the BLK/WHT with the red multimeter test lead.
Ground the black multimeter test lead directly on the battery negative (-) terminal.
With the Key On Engine Off (KOEO), the RED wire should have 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: The BLK/WHT wire is feeding the heated O2 sensor with 10 to 12 Volts DC. Good, since this confirms that the heated oxygen sensor's heater element is getting power.
The next step is to make check that the black (BLK) wire, of the O2 sensor engine wiring harness connector, is feeding Ground to the heater element. For this test, go to: TEST 2: Verifying The Heater Element Is Getting Ground.
CASE 2: The BLK/WHT wire IS NOT feeding the heated O2 sensor with 10 to 12 Volts DC. Re-check that you're testing the correct wire and that the key is in the RUN position (but don't crank or start the engine) and re-test.
If you still don't see 10 to 12 Volts DC, then this test result tells you that the heated oxygen (O2) sensor itself IS NOT bad since without power, the heater element won't work.
Although it's beyond the scope of this article, the next step is to find out why this battery power is missing using a wiring diagram.