Speed, idle & inputs · Powertrain · generic (SAE-defined)
P0505: Idle Air Control System
P0505 means the engine computer cannot control idle speed as intended, usually because of a problem with the idle air control (IAC) valve or electronic throttle idle control.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0505 sets
The powertrain control module (PCM) sets a target idle speed and adjusts the idle air control valve, or the electronic throttle on newer vehicles, to reach it. If actual idle speed cannot be brought close to the target, or the IAC circuit is faulty, the PCM sets P0505. Targets and tolerances vary by manufacturer.
Common causes of P0505
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Dirty or sticking idle air control valve
- Carbon buildup in the throttle body
- Vacuum leak
- Failed IAC valve motor or wiring fault
- Faulty electronic throttle body on drive-by-wire vehicles
P0505 symptoms
- Check engine light on
- High, low, or surging idle
- Stalling at stops
- Hard starting
How P0505 is diagnosed
- Scan for related codes, including vacuum leak, throttle, and lean codes
- Check commanded versus actual idle speed and IAC counts in live data
- Inspect for vacuum leaks around hoses and the intake manifold
- Remove and inspect the throttle body and IAC passages for carbon
- Test the IAC valve and wiring for proper resistance and continuity
- Perform an idle relearn procedure after cleaning or repairs
Typical fixes for P0505
- Clean the throttle body and IAC passages
- Replace the IAC valve
- Repair vacuum leaks
- Repair wiring or replace the electronic throttle body
- Perform an idle relearn
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
Commonly reported on older Honda, Toyota, and Nissan models with separate IAC valves. Newer SAE wording is "Idle Control System"; definitions can vary by manufacturer.
Related codes & warning lights
P0505 FAQ
What does code P0505 mean?
P0505 means the engine computer cannot control idle speed as intended, usually because of a problem with the idle air control (IAC) valve or electronic throttle idle control. The standard definition is "Idle Air Control System."
Can I drive with a P0505 code?
Usually yes, but idle can be unstable and the engine may stall when coming to a stop or when the air conditioning turns on. Avoid heavy traffic if stalling occurs and repair it promptly.
Does cleaning the throttle body fix P0505?
It often helps, since carbon deposits restrict idle airflow. An idle relearn may be needed afterward so the computer adapts to the cleaner throttle.
Why does the engine stall with P0505?
If the idle control system cannot supply enough air at idle, especially with added loads like air conditioning, engine speed drops too low and the engine stalls.
Keep researching
More speed, idle & inputs codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.