If an engine's idle speed varies greatly, which sensor might be the first to investigate?

When an engine's idle speed is unstable or varies greatly, the Throttle Position Sensor is crucial to investigate first. The Throttle Position Sensor (TPS) monitors the position of the throttle plate, which controls the air entering the engine. If the readings from the TPS are inaccurate or if the sensor is malfunctioning, it can lead to an improper air-fuel mixture being delivered to the engine, resulting in unstable idling characteristics.

Investigating the TPS makes sense because this sensor directly affects how much air is entering the engine during idle conditions. If the throttle plate is improperly positioned or if the sensor is sending incorrect signals to the engine control unit (ECU), it can cause erratic idle speeds or stalling.

Other sensors, while important, do not have an immediate impact on idle quality in the same direct way as the TPS. For instance, the Crankshaft Position Sensor is more related to timing and engine speed but may not specifically indicate idle variations. The Brake Pedal Position Sensor is primarily related to vehicle dynamics when the brake is applied. The Cylinder Head Temperature Sensor monitors engine temperature and, while it’s important for engine management overall, it will not directly cause variations in idle speed like a faulty TPS can.