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Oxygen sensor or O2 sensor The oxygen sensor or O2 sensor is used to sense the exhaust for rich or lean condition. The oxygen sensor produces it's own voltage ranging from 0 to 1 volt. The computer receives the signal and adjusts injector on time to compensate. Voltage above .450v is rich and voltage below .450v is lean. The sensor should switch regularly above and below .450v. If the oxygen sensor stays below .450v engine is running lean or sensor is bad or shorted to ground. If the oxygen sensor stays above .450v engine is running rich or oxygen sensor is bad or shorted to voltage. The oxygen sensor behind the catalytic converter monitors the converter effectiveness. Because it is burning any excess hydrocarbons it will use up the oxygen in the exhaust making it show higher voltage with less fluctuation then the front oxygen sensor. If the catalytic converter is bad the rear O2 sensor will fluctuate the same as the front O2 sensor and set a code for inefficient catalytic converter.
Engine temp sensor The engine temp sensor is mainly used to help the engine when it is cold. The temp sensor acts like a choke that was used on older cars to make them run rich when cold. Most temp sensors receive a 5v reference from the computer which is grounded through the sensor. As the engine warms up the resistance in the sensor drops, reducing the voltage at the sensor. Voltage ranges from about 4v on a cold engine to .5v on a fully warm engine. If the sensor does not drop in voltage the problems could be thermostat stuck open, or connection at the sensor or poor ground on the ground wire.
TPS or Throttle Position Sensor The throttle position sensor takes the place of the accelerator pump on the old carburetors. The TPS tells the computer that the throttle is being opened or closed to give a momentary change in fuel delivery. If the TPS has a glitch it can cause a hesitation or a jerking action. The best way to test the TPS is with a lab scope to see if there is a drop out of voltage as the sensor is moved, volt meters are not fast enough to catch most glitches. Voltage will range from about .5v with throttle closed to about 5v with throttle wide open.
Manifold air pressure sensor or MAP sensor The MAP sensor takes the place of the power valve in the old carburetor systems. When the engine has a load applied it must run richer. The MAP sensor is attached to the manifold vacuum to give a signal to the computer when there is a change in engine load. The voltage usually ranges from 5v when there is no vacuum to less than a volt when vacuum is high. Ford uses a different type of sensor giving a wave form measured in hertz. About 155hz with no vacuum to about 104hz when vacuum is high.