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Copyright - Things That Go - 2016

Things That Go

Now a few of the little bits of information about how the oxygen sensor is used not only for the operation of the engine, but also in diagnosing engine drivability and Check Engine light problems.

The oxygen sensor from now on will be referred to as the O2 sensor.

The O2 sensor has a warm up time before its output can be considered reliable. Early O2 sensors were totally dependent on exhaust heat to reach operating temperature. As emission standards tighten manufacturers were required to add a heater to the sensor so the sensor would reach operating temperature quickly so the PCM could control the emissions sooner. Just like a carbureted engine using a choke to richen the mixture during warm up, the fuel injected engine also needs a richer mixture. Getting the O2 sensors up to temp sooner means the PCM can reduce emissions sooner as the O2 sensors are considered reliable sooner. When the system has reached this point the system is considered operating in Closed-Loop. This means that the PCM is now making changes in the fuel mixture. Before this point, it is called Closed-Loop. In Closed-loop, with no feedback from the O2 sensors, the PCM is going by a pre-programmed fuel/spark map.

The manufacturer has determined that the engine when cold and using outputs from other sensors like RPM and engine temperature and throttle position to name a few, will require a certain amount of fuel. This will be more than ideal but they are concerned that the engine must produce enough power to drive well while warming up.

Using Oxygen Sensors to diagnose problems

Other FYI bits are that the PCM will actually cut off the fuel injectors when coasting to further reduce emissions and improve fuel economy. If you are driving a vehicle with a manual transmission you may actually feel when this happens. If you are above about 1200 RPM and remove your foot from the accelerator, several seconds later you may feel a slight increase in deceleration. Once the RPM is low enough the PCM will activate the injectors so the engine won’t stall as it nears its idle.

These codes, and others can be caused by inadequate fuel delivery.

The operation of the O2 sensor is such that it’s best suited for light operation providing information to the PCM to maintain the ideal 14:1 ratio. When you demand more power from the engine a richer fuel mixture is required. Again, the manufacturer has pre-programmed in a map of settings that the PCM uses, ignoring the output of the O2 sensors. Only a few European cars at this time use an O2 sensor output for idle/cruise and power mode.


If we go back to our graph of the O2 voltage we will see the voltage jump up to the maximum above ¾ throttle and stay there until the accelerator is returned to less than 3/4 throttle. Even though the PCM is ignoring the O2 voltage at this time, the sensors are still producing voltage. The richer mixture would cause the sensor to produce more voltage than it’s designed to do.

OBD2 Scanner

Misfire codes - P0300, P0301, P0302, P0304, P0305, P0306, P0307, P0308

Lean codes - P0171, P0174

How can this information help with diagnosing problems?


If we floor the gas pedal and the O2 voltage does not max out at .9 volts and stay there we know that the O2 sensor is reporting a lean condition. This may be caused by a weak fuel pump, a clogged fuel filter of a damaged fuel line. It may also be cause by restricted injectors depending on the type of injection system. If the voltage is maxed out above ¾ throttle we know that fuel supply is not the problem.

If the O2 voltage is above .9 volts when above ¾ throttle whatever issue you are having it is not caused by inadequate fuel supply.

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