Finding O2 Signal

Please turn the engine OFF before reaching into the engine compartment! Locate the O2 sensor being tested.

• Insert a pin into the output wire on the back of the O2 sensor connector to contact the signal wire. If you do not know which wire is the output wire, you must test them all. If there are 2 wires of similar color, test those LAST, as they are most likely the heater supply lines.

• Connect the positive probe of a voltmeter to the signal lead.

• Connect the negative probe of the voltmeter to ground.

• Start the engine and monitor the voltage (DC milivolts) as the engine warms up.

• When the engine is cold, the narrow band O2 sensor should output 0.1V to 0.2V and the wide band will put out around 5 volts.

• After about 2 minutes (summertime), the engine should reach operating temperature and the narrow band sensor output should fluctuate between 0.1V and 0.9 volts and the wide band between 3 and 5 volts, and the computer will run in closed loop mode.

• If your signal never fluctuates rapidly, the O2 sensor could be faulty.

Chrysler and Jeep Vehicles Checking your O2 sensor voltages can be tricky on these vehicles, as some of them add higher voltage to the sensor signal when the sensor is not at full operational temperature.

For this reason, it is important to test the O2 signal voltage when the engine is HOT and when depressing the gas pedal.

If your signal never fluctuates between .1 and .9 volts, or 2.5 and 3.5 volts, the O2 sensor could be faulty.