Tech Stuff

Easy air-fuel and oxygen sensor diagnosis

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE A6 certified with a M.A. from Columbia University. In the automotive world he has been trained by Technicians Service Training and Automotive Technician Training Services. Car Clinic’s facility is fully equipped with state-of-the-art factory-level equipment and services American, European and Asian vehicles, including diesels and hybrids.

Vehicles diagnosed by Craig Truglia and Alex Portillo. Contributions made by G. Truglia, Kevin Quinlan and Adam Varney.

Some technicians who have worked in this business for years often still get confused by how to diagnose an air-fuel sensor or are not sure what to look at when diagnosing a rear oxygen sensor. In fact, when I started in this business (which is not that long ago) I was told that there is no way to diagnose an air-fuel sensor with a high level of certainty. Let me set the record straight: there are several ways that you can diagnose any air-fuel or oxygen sensor and be confident that you will be doing the right repair.

The basics

Why do we even have these sensors? O2 and air-fuel sensors are the vehicle’s personal emissions analyzer. These sensors measure how rich or lean the exhaust is.

Air-fuel and oxygen sensors work in tandem, before and after the catalytic converter. The PCM compares the readings in order to analyze catalytic efficiency, and whether the vehicle is running rich or lean.

We will get into diagnosing catalytic efficiency by looking at the rear oxygen sensor later, but first let’s make sure that we understand how oxygen and air fuel sensors regulate fuel control on a vehicle.

So, when the air fuel or oxygen sensor senses a rich fuel mixture in the exhaust, the PCM takes that information and then tries to do the opposite to make a fuel mixture that is perfect (called “Lambda”) by sending fuel trims in the opposite direction.

Because these sensors go bad at a relatively high frequency, it is important to understand how they should work and what approach we should take with their diagnosis.

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