P0505 Idle Control System

Investigate soon

The ECU has logged a fault on P0505. The definition below is the exact wording the standard uses for this code.

Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

The definition

P0505: Idle Control System.

P0505 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
Check the connector at the part for corrosion, and that it is fully seated.
Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

Idle isn't controlled by a separate valve on most modern VAG petrol engines - the ECU holds idle by nudging the electronic throttle open and closed, helped by ignition timing. P0505 means it couldn't keep idle where it wanted it, or the idle control has run out of adjustment in one direction. In practice that nearly always means the engine is getting air the ECU doesn't know about, or isn't getting the air it expects through a gummed-up throttle. Only older engines with a mechanical throttle cable have an actual idle air control valve that can be replaced.

Most likely causes, in order

Unmetered air leaking in. Crankcase breather and PCV hoses go hard and split on these engines, PCV diaphragms tear, and intake boots and gaskets leak with age. A smoke test finds it in minutes, and you'll often see lean running or fuel trim codes stored alongside this one.
Carboned-up throttle body. Deposits build up around the throttle plate, so a given commanded position no longer flows the air the ECU is expecting. Clean it carefully, then run the throttle adaptation (basic setting) with a scan tool afterwards.
Throttle body not adapted, or faulty. After a battery disconnect, a clean or a replacement, many of these throttle bodies need their adaptation running before idle behaves properly. Look for throttle position codes stored alongside before condemning the unit itself.
Sticking idle valve (older engines only). Where a separate idle stabilisation valve is fitted, it can gum up and stick. Removing and cleaning it is well worth trying before buying a new one.

Worth knowing: Cleaning the throttle body without running the adaptation afterwards can leave the idle worse than when you started - and check the breather and PCV hoses first, because unmetered air is one of the most common root causes here and among the cheapest to put right.

Reading the code is the easy part

A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.

VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.

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Common questions

What does P0505 mean?

P0505 means Idle Control System. It is logged when the ECU sees this circuit outside its expected operating window.

Can I drive with P0505?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.