The Quick Answer above is the generic definition. What follows is the model-specific detail for the Honda models that log P0509.
DTC/P0509·Honda
P0509 on Honda: Causes, Symptoms and Fix Cost
Quick Answer
Moderate SeveritySafe to drive (short-term)Honda
P0509 on a Honda means the ECU is detecting voltage on the idle air control valve circuit above the normal range. The usual causes are an open circuit (broken wire, disconnected connector, or failed valve coil), or a wire shorted to a power source. P0509 is the inverse of P0508 and applies mostly to vehicles with a discrete IAC valve.
Typical repair cost, all makes€25 - €350
Connect any standard Bluetooth OBD2 adapter to your Honda and Skanyx tells you what P0509 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
Get Skanyx freeWhat does P0509 mean on a Honda?
Honda Civic
2001-2011The R18A Civic is drive-by-wire and has no separate idle air control valve, so a genuine P0509 on one of these points at the electronic throttle circuit or its wiring rather than at a valve coil. On older Civics that do use a valve, an open coil winding is the usual cause and the idle air adaptation needs resetting with the battery-disconnect relearn afterwards.
What causes P0509 on a Honda?
Beyond the generic causes listed on the main P0509 page, these are the patterns noted for Honda models:
Honda CivicElectronic throttle circuit or wiring fault on drive-by-wire R18A
Honda CivicConnector lock tab broken during previous intake service
How to diagnose P0509 on a Honda with OBD2
Follow these steps to pinpoint the root cause of P0509:
- Inspect the IAC valve connector for a fully seated lock and intact terminals. A common cause is a connector that was disturbed during recent intake work and never properly re-clicked
- Measure resistance across the IAC coil with the valve unplugged. An infinite reading confirms an open winding inside the valve
- Verify continuity of each control wire from the valve connector to the ECU pin per the wiring diagram. An open in either wire produces P0509
- With the ignition on and the valve unplugged, measure voltage on each connector pin to ground. A constant 12 V where the ECU is supposed to be pulsing indicates a short to power
- If wiring and valve are good, suspect the ECU driver circuit. Confirm with a known-good ECU or specialist scope before replacing the ECU
How much does P0509 cost to fix on a Honda? (EUR)
Estimated repair costs for P0509 in EUR, compiled across makes. Honda parts and labour usually land towards the lower end of each range.
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Codes related to P0509
These codes are often seen alongside P0509 or point to related issues, on any make:
Looking for the full P0509 reference (all makes, full diagnosis flow, complete repair cost matrix)?
See the main P0509 guideSkanyx · Honda
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