Oxygen Sensors Keep Fuel Mixture In Check

By Larry Carley

Most late-model O2 sensors and A/F sensors should last upward of 100,000 miles, but some don't make it that far.
Everybody knows what an oxygen sensor does, right? It senses oxygen in the exhaust, it gives the PCM a rich or lean signal as to what the fuel mixture is doing, and on late-model vehicles with Air/Fuel (A/F) sensors, it even tells the PCM the exact air/fuel ratio.

When an oxygen sensor reports a rich fuel mixture, the PCM responds by reducing the amount of fuel delivered. It does this by shortening the duration (on time) of the fuel injector pulses. If an oxygen sensor reports a lean fuel mixture, the PCM responds by increasing fuel delivery, and lengthens the on time of the fuel injector pulses. This results in a constantly changing fuel mixture that varies between rich and lean, with the average being right around the ideal air/fuel ratio of 14.7 to 1.

On most inline four- and six-cylinder engines, a single O2 sensor or A/F sensor is located in the exhaust manifold (although BMW and some other European applications use two). On V6, V8 and V10 engines, as well as horizontally opposed four and six-cylinder engines (such as Subaru and Porsche), a separate O2 sensor or A/F sensor is used for each cylinder bank. On some vehicles, such as a Ford F150 with a 300 six-cylinder engine, two O2 sensors are used in the one exhaust manifold (one to monitor the first three cylinders, and the other to monitor the rear three cylinders).

On 1996 and newer vehicles with OBD II, a “downstream” O2 sensor is mounted in or behind the converter to monitor catalyst efficiency. If the efficiency is dropping off, it will set a fault code for the converter and turn on the Check Engine light. This monitor won’t run, however, if any of the O2 sensors (upstream or downstream) are not working properly and have set a code.

Most late-model O2 sensors and A/F sensors should last upward of 100,000 miles, but some don’t make it that far. Internal coolant leaks (bad head gasket) or an oil consumption problem (worn valve guides or valve guide seals, or worn or broken piston rings) can sometimes foul an O2 sensor and cause it to fail.

The onboard diagnostic system monitors the performance of the O2 sensors every time the vehicle is driven. It checks the O2 sensor heater circuits to make sure they are warming up the sensors when a cold engine is first started, and they look at what the sensors are telling the PCM about the air/fuel mixture to see if it makes sense or is out of range. No response from an O2 sensor, or a sluggish response is a sure sign the sensor has died and needs to be replaced.

An O2 sensor-related code doesn’t necessarily mean a sensor has failed in some cases. If there’s a heater code, the fault might be outside the sensor (a bad wiring connection or a heater circuit relay, fuse, power supply or control problem). The other possibilities should be ruled out before condemning the heater circuit inside the sensor.

O2 sensor codes that indicate a rich or lean condition often have nothing to do with the sensor itself. It is merely reporting what is happening upstream of the sensor in the combustion chamber. The problem may be fuel, ignition or compression-related — all of which require further diagnostics to pinpoint the real cause of the code.

O2 sensors can fail, and when they do it takes the PCM out of closed loop fuel feedback control and puts it into a preprogrammed mode that often causes the engine to run rich, pollute and waste fuel. O2 sensor problems are a common cause of emission test failures (both OBD plug-in tests and loaded mode tailpipe tests).

New O2 sensors can restore like-new engine performance and fuel economy, and are often recommended when the 100,000 mile spark plugs are replaced for preventive maintenance.

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