P0505

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Driveable if idle is stable. If it stalls when coming to a stop or when the air conditioning engages, get it fixed before driving in traffic.

Urgency

Diagnose soon

Will it pass emissions

No

If ignored

Repeated stalling, hard starting, and a possible no-start when the condition worsens. High idle causes creep in drive and accelerates brake and transmission wear. On systems where the PCM keeps adapting around the fault, drivability degrades gradually until it becomes a running problem rather than an idle one.

Check engine light

Solid

Clearing the light

A completed drive cycle is normally required before the monitor re-runs and the light can clear.

• Idle speed too high, too low, or unstable
• Possible stalling at idle or after cold start

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. Whether lean codes such as P0171 or P0174 are also stored

   Lean codes alongside P0505 make an unmetered air leak the leading candidate by a wide margin, and change the whole diagnostic order.

2. Whether the throttle body was recently cleaned or the battery recently disconnected

   Most electronic throttle systems need an idle relearn procedure after either. Skipping it produces exactly this code and symptom with nothing actually broken.

3. The PCV hose, intake boot after the mass air flow sensor, and brake booster hose

   These are the three most common unmetered air leaks and all three can be inspected by hand in a few minutes.

4. Idle air control counts or commanded throttle position in live data

   Tells you which end of its authority the PCM has run out of. Pinned low means it is fighting extra air. Pinned high means it cannot get enough.

5. Whether the condition is cold-only, warm-only, or constant

   Cold-only points at a valve sticking when cold or a coolant temperature sensor fault. Constant points at a leak or a restriction.

1. Read all codes and record idle adaptation values

   Tools OBD-II scan tool

   Pull every stored and pending code. Note fuel trims at idle and at higher RPM. Fuel trims strongly positive at idle but normal at cruise is the classic signature of a vacuum leak, because the leak is a large proportion of a small airflow at idle and a small proportion at speed.

   Expected

   Fuel trims within roughly plus or minus ten percent. Idle correction values not pinned at either limit.

2. Watch the idle control PID against actual RPM

   Tools Scan tool with live data

   With the engine fully warm and idling, observe idle air control steps or commanded throttle angle alongside actual and target RPM. Then load the engine with the air conditioning and by selecting drive, and watch whether the PCM can hold target.

   Expected

   The correction value moving smoothly and holding target RPM under each load, without saturating at a limit.

3. Smoke test the intake for unmetered air

   Tools Smoke machine

   Introduce low-pressure smoke into the intake tract with the throttle closed and inspect everywhere downstream of the mass air flow sensor: intake boot seams, PCV hoses and valve, brake booster hose and booster body, intake manifold gasket, throttle body gasket, and any vacuum port caps. This test finds leaks that parts replacement cannot. Pay particular attention to the brake booster and its hose - a leak there is both an idle fault and a brake-assist fault.

   Expected

   No smoke escaping anywhere downstream of the mass air flow sensor.

4. Inspect and clean the throttle bore and idle air passage

   Tools Throttle body cleaner, lint-free cloth, socket set

   Remove the intake ducting and look at the throttle plate edges and the bore behind it. Carbon buildup on the sealing edge changes the airflow at closed throttle enough to defeat idle control. On systems with a separate idle air control valve, remove it and check the pintle and its passage for carbon and for the pintle sticking in its bore.

   Expected

   A clean bore and plate edge, and an idle air control pintle that moves freely through its full travel.

5. Test the idle air control valve electrically

   Tools Digital multimeter, bidirectional scan tool

   On vehicles with a discrete valve, measure winding resistance with the valve disconnected and the key off, comparing each coil against the other coils in the same valve on stepper designs. Then command the valve with a scan tool and confirm it responds. Resistance specifications vary by manufacturer, so use service data.

   Expected

   Each winding reading in line with the others in the same valve, and the pintle responding visibly to commanded position.

6. Verify throttle position and pedal position correlation on electronic throttle systems

   Tools Scan tool with live data

   Compare the two throttle position signals and the two accelerator pedal signals against each other through their full sweep. Disagreement between redundant sensors defeats idle control and will often set correlation codes alongside P0505.

   Expected

   Redundant sensors tracking each other consistently through the full range with no dropouts.

7. Check supporting inputs the idle strategy depends on

   Tools Scan tool, digital multimeter

   Confirm the coolant temperature sensor reads correctly - a sensor reading falsely cold keeps the idle target elevated and the mixture rich. Check the mass air flow reading at idle for plausibility, and verify the air conditioning and power steering load signals reach the PCM. Also confirm the throttle cable or linkage is not holding the plate slightly open on cable-throttle vehicles.

   Expected

   Coolant temperature matching ambient after an overnight soak and climbing normally. Mass air flow plausible for the displacement at idle.

8. Perform the idle relearn and confirm over a drive cycle

   Tools Scan tool, service information

   After any throttle body cleaning, valve replacement, or battery disconnect, run the manufacturer's idle relearn procedure. The procedure is vehicle-specific - some require a scan tool reset, others a defined key-on and idle sequence. Then verify idle holds through cold start, warm idle, air conditioning on, and in gear.

   Expected

   Stable idle at target under every load condition, and no return of the code.

Cleaning and relearn is often the whole repair and is cheap, so it is a reasonable first step once a smoke test has ruled out leaks. Electronic throttle bodies are the expensive outcome and are worth confirming properly first, because many are replaced on suspicion. Intake manifold gasket work varies widely by engine layout.

• Replacing the idle air control valve or throttle body when the real fault is a vacuum leak the smoke test would have found in minutes.
• Cleaning the throttle body on an electronic throttle vehicle and not performing the idle relearn, then blaming the cleaning for the symptom that follows.
• Scrubbing a coated electronic throttle bore with aggressive cleaner and abrasive pads, which removes the coating and makes the idle problem permanent.
• Ignoring positive fuel trims at idle, which are the strongest single clue available on this code.
• Overlooking a sticking throttle cable, a floor mat against the pedal, or a cruise control cable holding the plate slightly open on older vehicles.
• Missing a coolant temperature sensor reading falsely cold, which keeps the idle target high with the idle control system working correctly.
• Treating a leaking brake booster or booster hose as an idle problem only - it also degrades brake assist and should be repaired on that basis alone.

• Throttle Body
• Idle Air Control Valve

• P0504Brake Switch "A"/"B" CorrelationSeverity level 5 of 5: Stop driving.critical
• P0506Idle Air Control System RPM Lower Than ExpectedSeverity level 3 of 5: Diagnose soon.moderate

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

Written and reviewed in-house. See our editorial policy.

Review status

Reviewed

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