How to Do a Voltage Test on a Oxygen Sensor


1. Park your car in a safe place and warm the engine to operating temperature. This will kick the oxygen sensor into operating range. Make sure the emergency brakes are applied, and block the front and rear wheels with wood blocks or bricks.
2. Open the hood and locate the oxygen sensor following the front exhaust pipe connected to the exhaust manifold. The sensor is threaded into the front pipe between the manifold and the catalytic converter. If your vehicle has two sensors, you will find the second one on the exhaust pipe between the catalytic converter and the muffler. You may need to raise the vehicle to gain access to this sensor. Raise the car using a floor jack and safely support it on a jack stand.
3. Set your voltmeter to millivolts (mV) and back probe the O2 sensor signal wire with the meter's red lead; connect the meter's black lead to a good ground on your vehicle. A regular oxygen sensor only has one wire, while a heated oxygen sensor has three wires: signal, heater and ground. Make sure you are probing the signal wire. If necessary, consult your vehicle service manual to identify this wire.
4. Read the voltage from the sensor for about one minute with the engine running. You should get a fluctuating output voltage between 0.2 and 0.8 volts. with an average of about 0.5 volts. A voltage reading that remains low (close to 0.2 v) or high (close to 0.8) means the engine has a lean or high fuel ratio, respectively. Either the sensor is not responding, a fuel injector is leaking or clogged, or some other problem is affecting the air/fuel ratio.
5. Disconnect a large vacuum hose form the engine. Keep the engine idling and at operating temperature. The sensor's output voltage should go down to a lean condition--close to 0.2 volts. Connect the vacuum hose.
6. Snap the engine throttle open and close while reading the sensor's output voltage on the voltmeter. The sensor should cycle rapidly with the extra air.
7. Block the air inlet at the air intake system using one or more shop rags. The sensor's output voltage should go up to a rich fuel condition--close to 0.8 volts. Remove the shop rags from the air inlet.
8. Replace the oxygen sensor if its output voltage does not react to these tests.