What Are Engine Sensors, and What Do They Do? 

The engine computer, or Electronic Control Module (ECM) and its associated sensors control almost every aspect of engine performance. The following glossary of terms defines 17 of the most common sensors and other components found on a modern, computer-controlled automobile.

• Coolant Temperature Sensor (CTS)
  Measures the temperature in the cooling system, so the computer may make adjustments based on the engine's operating temperature. Can also control the dashboard warning light.
   
• Crankshaft or Camshaft Position Sensor
  Monitors the rotation of the engine and tells the computer exactly when to trigger the fuel injectors or the ignition spark.
   
• Detonation (Knock) Sensor
  Listens for engine "ping" so the computer can retard spark timing, and thereby reduce emissions and overheating.
   
• Electronic Control Module (ECM/Computer)
  Controls spark timing, fuel delivery and emission controls. Continuously receives signals from sensors and input devices on or near the engine; send control signals to valves, controllers and other output devices. Stores trouble codes and warns driver when service is needed.
   
• Exhaust Gas Recirculation (EGR) Valve
  Recirculates a measured amount of exhaust gas into the engine's air intake to lower combustion temperatures and reduce emissions, especially NOx.
   
• EGR Valve Position Sensor
  Detects the opening of the EGR valve so the computer can make adjustments to optimize performance.
   
• Fuel Injector
  Injects fuel into the intake manifold. The computer tells the injector exactly when and how much fuel to inject in order to produce the needed amount of power.
   
• Idle Speed Control Actuator
  Adjusts the idle speed as dictated by the computer, to prevent idle fluctuations and keep emissions low.
   
• Manifold Absolute Pressure (MAP) Sensor
  Reads a change in manifold pressure. The computer uses this information to adjust timing advance and air/fuel ratio.
   
• Mass Air Flow (MAF) Sensor
  Measures the mass of the air drawn through the engine's air intake, so the computer can compensate for altitude and temperature changes.
   
• Positive Crankcase Ventilation (PCV) Valve
  Recirculates partially burned gases from the crankcase into the combustion chamber to improve fuel economy and reduce emissions while preventing the buildup of sludge and corrosion.
   
• Oxygen Sensor
  Measures the percentage of oxygen in the exhaust, and tells the computer whether the fuel/air mixture is too rich or too lean.
   
• Throttle Position Sensor (TPS)
  Monitors the position of the accelerator pedal and throttle linkage so the computer can make accurate air/fuel mixture adjustments.