Tech Stuff

Advanced air-fuel and oxygen sensor diagnosis

Portillo is the head technician of Car Clinic, a state-of-the-art automotive repair facility in Mahopac, N.Y. He has been trained by Automotive Technician Training Service and is TST certified. Portillo’s real-world, in-depth diagnostic articles appear in Auto Service Professional on a regular basis.

Not long ago, Auto Service Professional covered all of the basics concerning how to diagnose common oxygen and air-fuel sensor codes (see the March/April 2012 issue). This time, we are going to discuss more advanced sensor diagnosis so you can catch those really tough problems that are not simply a matter of checking the heater circuit and having a specification.

Very brief review

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.

Figure 1: When the air-fuel sensor detects rich exhaust, it reports this to the PCM which then takes away fuel to make the air-fuel mixture normal. This is why rich exhaust creates negative fuel trim and vice versa.
<p>Figure 1: When the air-fuel sensor detects rich exhaust, it reports this to the PCM which then takes away fuel to make the air-fuel mixture normal. This is why rich exhaust creates negative fuel trim and vice versa.</p>

When the air fuel or front 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. Air-fuel sensors reflect a lean condition when their voltage increases and a rich condition when their voltage decreases. Oxygen sensors work the opposite way, with an increase over 450 mV reflecting a rich fuel mixture and a decrease below that number a lean fuel mixture (see Figure 1).

Post-cat oxygen sensors, when good, feature a steady voltage usually between 500 to 700 mV. If it zigzags, the catalytic converter is highly suspect.

Quick air-fuel sensor check. Are you convinced the A/F sensor is stuck lean or rich, but don’t have the right specification? Until advanced milliamp clamps are mainstream where you would be looking for a specification of 0 amps (+ or –0.03 mA), you will have to either put a digital multimeter in series hooked up in the amps port. This is time consuming and putting the meter in series between the wrong wires can fry the PCM. A better method is to stick an emissions analyzer in the tailpipe. If the rear oxygen sensor has elevated mV (something in the 800 mV range) and Lambda is rich, you likely have an A/F sensor stuck lean. An A/F sensor stuck rich is much more rare, but theoretically can be approached in the same way (low rear oxygen sensor voltage and lean Lambda).

Tough P003X oxygen sensor codes. You get an oxygen sensor code, the repair is almost always an oxygen sensor, right? Well, if you have a code in the P003X (X equaling 1 through 9) range, you should expect that the vehicle has the wrong sensor or a module problem. This can be best illustrated in a case study. We received a phone call from a local shop about a 2003 Toyota Camry 2.4L four cylinder 2AZ-FE California emissions vehicle with a strange P0031 Oxygen Sensor Heater Control Circuit Low B1S1 (see Figure 2). This is how the story goes:

Figure 2: 2003 Toyota Camry with a P0031.
<p>Figure 2: 2003 Toyota Camry with a P0031.</p>

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