P1509 on BMW vehicles indicates that the engine control module (DME) has detected an electrical fault in the control circuit for the idle speed control valve's opening solenoid. The computer monitors the circuit for expected voltage or current and flags the code when the signal falls outside the valid range, which can prevent the valve from properly regulating idle speed.
Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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