Advanced air-fuel and oxygen sensor diagnosis Time-saving advice on catching the really tough problems By Alex Portillo
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 certifed. Portillo’s real-world, in-depth diagnostic articles appear in Auto Service Professional on a regular basis.
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ot 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 specifcation.
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.
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 Very brief review converter is highly suspect. Air-fuel and oxygen sensors work in Quick air-fuel sensor check. Are you tandem, before and after the catalytic convinced the A/F sensor is stuck lean or converter. The PCM compares the readings rich, but don’t have the right specifcain order to analyze catalytic effciency, and tion? Until advanced milliamp clamps are whether the vehicle is running rich or lean. mainstream where you would be looking When the air fuel or front oxygen sensor for a specifcation of 0 amps (+ or –0.03 senses a rich fuel mixture in the exhaust, mA), you will have to either put a digital the PCM takes that information and then multimeter in series hooked up in the amps tries to do the opposite to make a fuel mix- port. This is time consuming and putting ture that is perfect (called “Lambda”) by the meter in series between the wrong sending fuel trims in the opposite direction. wires can fry the PCM. A better method is Air-fuel sensors refect a lean condition to stick an emissions analyzer in the tailwhen their voltage increases and a rich con- pipe. If the rear oxygen sensor has elevated dition when their voltage decreases. Oxygen mV (something in the 800 mV range) and sensors work the opposite way, with an Lambda is rich, you likely have an A/F increase over 450 mV refecting a rich fuel sensor stuck lean. An A/F sensor stuck rich
13 | January/February 2013
