P0509 indicates that the engine control module (ECM) has detected a voltage higher than the expected range in the idle air control (IAC) circuit. This typically points to an electrical fault such as a short to battery voltage, a failed IAC valve, or damaged wiring.
The IAC valve itself is the most common failure point. An internal short to power or a mechanically stuck pintle can cause the circuit voltage to read high.
A chafed, pinched, or melted wire in the IAC circuit can make direct contact with a 12V source, raising the signal voltage above the expected range.
Poor connections at the IAC valve can create intermittent high-resistance or voltage spikes, though this is less common for a 'high' code.
In very rare cases, an internal failure of the engine control module can cause a high voltage reading. This should only be considered after all other possibilities are ruled out.
It is not recommended because the high idle or stalling can be a safety hazard, especially in traffic. The vehicle may also fail an emissions test.
Cleaning may help if the valve is sticking due to carbon buildup, but P0509 is a circuit high code, meaning an electrical issue. Cleaning alone rarely fixes an electrical short.
No. Many drivers are told the PCM is faulty, but that is almost never the case. The problem is almost always the IAC valve or its wiring. Only replace the PCM after thorough diagnosis.