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Q: O2 Sensors

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I am getting a check engine light. Took to dealer and they said all four O2 sensors came up on diagnostic. They said unlikely that all fail at once reset and told me to drive it. Chk eng light came back on a mile down the road. What should I check?

My car has 170000 miles.
My car has an automatic transmission.

A: As you may know the oxygen sensors, which a...

As you may know the oxygen sensors, which are part of your emissions system, read exhaust gases to determine whether or not the appropriate amount of oxygen is present in the exhaust. Based upon this reading, this information is relayed to the car's engine control unit (ECU), which then makes a determination as to whether or not to add more or less fuel to the engine. When too much or too little oxygen is present, this will cause check engine light to come on. Too much oxygen may be present in the exhaust gas due to things like an exhaust leak, a vacuum leak, a faulty mass air flow sensor (causing a lean fuel condition) or faulty fuel injectors. Too little oxygen in the exhaust gas may be caused by things like a clogged air filter, a failing mass air flow sensor (causing too much fuel to be introduced to the motor), bad fuel injectors or a clogged catalytic converter. When the O2 sensor is not working properly, this will cause false readings to be relayed to the computer causing inaccurate adjustments to be made to the intake and fuel system in your engine. If you want assistance in diagnosing what is the cause of this symptom, a certified YourMechanic technician can come to your location to perform a Check Engine Light is on Inspection on your vehicle and suggest any repairs.

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