- 1). Plug the code scanner into the on-board diagnostics (OBD) port located under the dash to the left of the steering column. Turn the ignition switch to the “on” position without starting the engine.
- 2). Depress the “Read” key on the scanner. Make a note of the codes displayed. Cross-reference the code with the code sheet for an explanation of the code. The code identifies which of the four oxygen sensors has an irregular signal.
- 3). Check the sensors individually with a voltmeter. The code scanner usually gives enough information to deduce which sensor has failed, however, sensors can be checked manually with a helper. Each sensor has a round electrical plug. When holding the plug so it is being viewed from the harness to sensor side, notice there are four wires. Locate the top of the plug, which in this case is the locking clip. This is the top. Reading left to right, the top two are numbered 1-2 respectfully, and the same for the bottom 3-4.
- 4). Connect the black voltmeter lead to a good ground and use the red lead with the wire probe to pierce the number 4 wire — the lower left wire.
- 5). Start the truck and allow it to warm up sufficiently to cause the sensor to go into closed loop. Have the helper hold 2,500 rpm while you observe the voltmeter. The voltage should be continuously rising and falling with the minimum voltage being 250mv and the maximum of 800mv. It should be fluctuating between three to five times per second. If there is little or no change in the voltage, the sensor is bad.
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