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Advanced O2 sensor diagnostics Tracing sensor wiring and checking for ‘lazy’ sensors
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ver the years we covered a lot of the basics concerning oxygen and air-fuel sensor diagnostics. This includes their basic operation, the differences between oxygen and air-fuel sensors, air-fuel ratio sensor voltage specifcations, the differences between zirconium and titanium dioxide oxygen sensors, and using emissions analyzers to verify Lambda. If we can understand all of the preceding, there is not much more that we need to know in order to test these sensors. However, we have not yet covered tracing O2 sensor wiring and checking for a “lazy” sensor. After reviewing the basics, we will move onto troubleshooting these more advanced oxygen sensor issues.
These sensors measure how rich or lean the exhaust is. The “front” sensors, located in front of the catalytic converter, are the ones that are used to determine fuel control. On a scan tool, the B1S1 and B2S1 are the front sensors, as the “S” means “side,” and “1” means that it is in front of “2,” which would be the rear sensor. The B stands for “bank,” and 1 and 2 indicates which bank of the motor is being referenced. Being that motors are in all different positions in different vehicles, the technician needs to look up the fring order on an information system in order to know which bank is 1 or 2. The front sensor is always on the same side as cylinder number one in the fring order. Understanding fuel control Why are vehicles designed this way? Oxygen and air-fuel sensors are the Obviously, if the exhaust goes through the vehicle’s personal emissions analyzer. converter, the emissions are cleaned up and this affects the signal. For this reason, the front sensors are located in front and are designed to be sensitive so that they can detect fuctuations in the air-fuel mixture. 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. The same is true when the oxygen sensor Figure 1: Depicted here is a graphic representation of how senses a lean fuel mixture in the PCM reacts when the front oxygen sensors detect an the exhaust. When this occurs, air-fuel mixture above or below Lambda.
14 | July/August 2015
