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Heated Lambda oxygen sensor
1) Sensor housing, 2)Ceramic support tube, 3) Electrical connections, 4) Protective tube with slots, 5) Active sensor ceramic, 6) Contact element, 7) Protective sleeve, 8) Heating element, 9) Clamp-type connections for heating element |
All Lambda (O2) sensors work the same way. The 3- (or 4-) wire units merely have a heater coil to get them up to operating temperature faster. With these, you have to know which wire is actually the signal wire. Assuming you can get this off the wiring diagram, you connect a digital voltmeter (DVM) between this wire and ground. After the car is warmed up, you should see the voltage jumping back and forth between about 0.2 and 0.9 volts. A steady reading usually indicates a bad sensor. The problem could be elsewhere, as the O2 sensor only gives the EFI system feedback about what it did to the mixture. Current systems use the sensor as a switch. They let the air to fuel mixture wander back and forth across the ideal point using the O2 sensor as a trip point detector. If something else in the system has it pegged on full rich, the O2 sensor will appear to give bad readings.
There is also a way to test the O2 sensor on the bench, out of the car. You clamp the hex nut part in a vise, connect the DVM between the signal wire and the vise, then heat the sensor tip with a propane torch. The voltage should rise to about 0.8-0.9 volts, falling off rapidly as you take away the flame. You have to heat the thing up cherry red, but once hot, it should respond rapidly to more flame or less. Slow or no response means a bad sensor.