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Engine Sensors Keep Everything In Check

Sensors are monitored by the Onboard Diagnostic (OBD II) system, which will usually set a code and turn on the Check Engine light when a sensor problem or failure occurs.



Engine sensors provide inputs for the Powertrain Control Module (PCM) so the engine computer can manage the fuel mixture, spark timing, emission functions and other control functions that are necessary to operate a modern vehicle. Without accurate inputs, the PCM can’t do its job, and engine performance, fuel economy and emissions will suffer. Sensors are monitored by the Onboard Diagnostic (OBD II) system, which will usually set a code and turn on the Check Engine light when a sensor problem or failure occurs.
Here are a few key sensors:

● Coolant sensor — Usually located near the thermostat
housing, it monitors engine coolant temperature so the PCM can change the fuel mixture and idle speed as the engine warms up. It’s input may also control the operation of the radiator cooling fan.


● Oxygen sensors — Located in the exhaust manifold(s), the O2 sensor provides a rich/lean air/fuel mixture feedback signal for the PCM so the fuel mixture can be adjusted for lowest emissions and optimum fuel economy. On V6, V8 and V10 engines, there is usually one O2 sensor in each cylinder bank. On most four and straight six engines, there is usually only one O2 sensor (unless the exhaust manifold is split into two parts).

A “downstream” O2 sensor located in or behind the catalytic converter monitors catalyst efficiency and assists with long-term fuel trim.

Many late-model engines use a more sophisticated “wide ratio” O2 sensor (Air/Fuel sensor) that provides a more exact indication of the air/fuel mixture for better fuel control.


Oxygen sensors age and may become sluggish or contaminated, causing a drop in fuel economy and an increase in emissions.

● Mass Airflow Sensor (MAF) — Located between the air filter housing and throttle, this sensor measures the volume of air entering the engine so the PCM can vary the air/fuel mixture. Problems here include air leaks that allow unmeasured air to bypass the sensor, or surface contaminants that make the sensor sluggish or read low. Cleaning a MAF sensor with aerosol electronics cleaner can sometimes restore normal operation.

● Throttle Position Sensor (TPS) — Mounted on the throttle body, this sensor monitors the position of the throttle shaft so the PCM knows when the engine is at idle (for idle speed control) and when it is accelerating (for fuel enrichment) or decelerating (to cut fuel delivery for better economy). The sensor also may be used to provide feedback for the electronic throttle control system. A bad TPS sensor may cause a flat spot or hesitation when accelerating, or cause idle or throttle control problems.


● Manifold Absolute Pressure (MAP) Sensor — Usually located on the intake manifold (or attached to the manifold by a hose), this sensor measures intake vacuum (and pressure on engines that are turbocharged or supercharged) so the PCM can determine engine load.

● Crank Position Sensor (CKP) — Located on the front or back of the engine, or in the side of the engine block, it monitors the relative position of the crankshaft for spark timing and injector timing. It is also used by the OBD II system to detect engine misfires. A bad crank sensor may prevent the engine from starting.

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