Circuit Test O2S - Oxygen Sensor

WARNING: This page is about a different car, the 1994 Mercury Tracer and 1994 Ford Escort. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Enter this test only when Code 15 (lean) or Code 17 (rich) is displayed during self-test procedures (KOEO or KOER). This test is intended to diagnose O2S circuit.

NOTE: Voltage that remains more than .55 volt indicates a continuously rich condition. Voltage that remains less than .55 volt indicates a continuously lean condition. Rich or lean conditions may also be an indication of another problem.

1) Check O2S Voltage Ensure engine is at normal operating temperature. Turn ignition off. Disconnect O2S connector. Measure voltage at O2S connector. Voltage should increase as engine speed increases and decrease as engine speed decreases; voltage should be 0.2-0.8 volt at idle. If voltage is as specified, go to next step. If voltage is not as specified, go to step 3).

2) Check O2S Circuit Turn ignition off. Install breakout box, leaving PCM disconnected. Measure resistance between O2S test pin at breakout box and O2S connector. If resistance is greater than 5 ohms, repair O2S wire. If resistance is less than 5 ohms, O2S circuit is okay. If no other problem or code is found, replace PCM.

3) Check O2S Circuit For Short To Ground Turn ignition off. Install breakout box (PCM disconnected). Disconnect O2 sensor. Measure resistance between breakout box O2S test pin (29). Resistance should be greater than 10,000 ohms. Measure voltage at breakout box O2S test pin (29). Voltage should be zero volts. If both readings are within specification, replace O2S. If readings are not within specifications, repair O2S wire to PCM.