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Soot and oil residue baked onto a throttle body, intake runner flap or its bore, until the moving part no longer travels where it is commanded. The module orders a position, measures what it got, and reports the shortfall — which reads as an actuator fault while the actuator is fine and the passage is dirty.

Direct-injection engines suffer most, and the reason is structural. Port-injected engines spray fuel onto the back of the intake valve, and that fuel washes deposits away continuously. A direct-injection engine injects into the cylinder, so nothing ever cleans the intake side — deposits simply accumulate for the life of the engine.

Short journeys accelerate it, because an engine that never reaches full temperature never burns deposits off.

Cleaning is usually the repair rather than replacement, and it is worth asking for explicitly — a throttle body or runner assembly is an expensive part to replace for a fault that a solvent and an hour would fix. Many vehicles need the throttle position relearned afterwards.