Engine Sensors — What the Computer Reads

The engine computer — the PCM — cannot see, hear, or feel the engine. It relies entirely on sensors to tell it what is happening. Each sensor converts a physical condition — temperature, pressure, position, speed — into an electrical signal the PCM can read. Understanding what each sensor measures and what happens when it fails is fundamental to diagnosis.

Measures engine coolant temperature. A thermistor — a resistor that changes resistance based on temperature. Cold engine — high resistance, low voltage signal. Hot engine — low resistance, high voltage signal. The PCM uses this to determine cold start enrichment, cooling fan activation, and transmission shift strategy. A failed ECT sensor that reads too cold causes the engine to run rich, waste fuel, and never go into closed loop fuel control.

Measures the volume and density of air entering the engine through the intake. A heated wire or film element inside the sensor housing is cooled by incoming air. The more air flows past, the more cooling occurs, and the sensor reports a higher airflow value. The PCM uses this to calculate exactly how much fuel to inject. A contaminated MAF sensor — from oil residue in an oiled aftermarket air filter for example — reads incorrectly and causes lean or rich fuel conditions. MAF sensors can be cleaned with dedicated MAF sensor cleaner spray. Never touch the sensing element with anything.

Tells the PCM how far the driver has pressed the accelerator pedal. On electronic throttle systems — drive by wire — a sensor on the accelerator pedal tells the PCM the driver's intent, and a sensor on the throttle body tells the PCM the actual throttle position. The PCM compares the two and commands the throttle motor. A TPS with a dead spot — a position where the signal drops out momentarily — causes a hesitation or stumble at that specific throttle position.

Measures intake manifold vacuum and pressure. At idle the manifold has high vacuum — typically 18 to 22 inches of mercury. At wide open throttle the vacuum drops to near zero. On turbocharged engines the MAP sensor also reads positive boost pressure above atmospheric. The PCM uses MAP data along with engine RPM to calculate engine load and determine fuel delivery and ignition timing. Some engines use a MAP sensor instead of a MAF sensor. Some use both.