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Testing O2 Sensor

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To check for a bad O2 sensor, set the multimeter to ohms. Connect the positive lead to the signal wire, and the negative lead to ground. Resistance will vary from model to model, but the test procedure is always the same. We’ll use a ’99 TJ with a 4.0 engine as an example. With the key off, a good O2 sensor should read between 4.5-7 ohms.