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Testing O2 sensors
I know of two ways to test an O2 sensor; On the car, and on the workbench. The workbench test provides more accurate results, but it requires more work.
Testing O2 sensors on the car
First, the engine must be fully warm and the thermostat must be properly functioning. Without a working thermostat, most cars won't go into closed loop mode. Attach the positive lead of a high impedance DC voltmeter to the oxygen sensor output wire. Leave the wire attached to the ECU. This means the connection will have to be back probed. Ground the voltmeter's negative lead. This may not work with a cheap voltmeter. That's because a cheap meter will load down the circuit and absorb the voltage that it's trying to measure. A good minimum spec for the voltmeter is 1,000,000 ohms/volt of DC voltage. Every digital voltmeter I've seen meets this spec, although I'm sure ultra-cheap digital voltmeters do exist. An analog voltmeter will probably have to be powered to work. Heated O2 sensors have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. With a heated sensor, I use a 15 or higher volt scale on the meter to determine which wire is the sensor output wire. Using a lower voltage scale could damage the meter. After figuring out which wire is the sensor output wire, I set the meter to look for 1 volt DC.
At that point I turn on the ignition key, without starting the engine. This should cause a change in voltage on the meter. If not, I double check the connections. Once I can see the voltage change, I start the engine and keep the revs above 2000 rpm for about two minutes. This will warm up the O2 sensor and hopefully get the ECU into closed loop. At idle, I'm looking for the voltage to go above and below 0.45 volts. If I see less than 0.2 and more than 0.7 volts and the value changes quickly, I'm done. The sensor has just passed all the testing that can be done while it's still on the car. If it gives a steady voltage, the O2 sensor is probably bad.
Testing O2 sensors on the workbench.
After removing the sensor from the car, I clamp the sensor in a vice. Some people use pliers or vice grips to hold the sensor, but with a torch involved, I'm more comfortable using a vice. Using the same type of DC voltmeter mentioned in the prior test, I clamp the voltmeter's negative lead to the O2 sensor case, and the positive to the output wire. Using a propane torch set to high, I use the inner blue flame tip to heat the perforated part of the O2 sensor. I'm looking for 0.6 volts within 20 seconds. If that works, I stop heating the O2 sensor. Now I should see a drop to under 0.1 volt within 5 seconds. Now, I heat the O2 sensor for two minutes and watch for a drop in voltage. If there's a voltage drop, the sensor is bad. Adding heat again should result in a rapid increase in voltage. If the sensor has made it this far, it's probably good.
If an O2 sensor can produce 0.9 volts or more when heated, drop below 0.2 volts within one second of having removed the flame, AND pass the two minute test, it's good.
If you find yourself replacing an O2 sensor that failed a test, you may want to perform the same test on the new O2 sensor. This will give you an opportunity to see how fast a functioning sensor responds.
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