P0505 – Idle Air Control System

P0505 means the ECU has detected a malfunction in the Idle Air Control (IAC) system — the system responsible for regulating engine idle speed. When this system fails, you may experience rough idle, stalling, fluctuating RPM, or an idle that’s too high or too low. The code triggers when the ECU cannot maintain the programmed idle speed despite adjusting the IAC valve or throttle control. In this guide, you’ll learn exactly what P0505 means, the symptoms, common causes, step-by-step diagnostics, and the repairs that typically fix it.

What Does P0505 Mean?

P0505 sets when the ECU detects that commanded idle control values do not match actual idle speed. On older vehicles, this involves the Idle Air Control (IAC) valve located on the throttle body; on newer drive-by-wire vehicles, the ECU controls idle speed via the electronic throttle body. If the ECU cannot achieve the desired idle RPM, it determines the idle control system has malfunctioned and triggers this fault.

Common reasons include carbon buildup in the throttle body, vacuum leaks, a sticking IAC valve, wiring problems, or malfunctioning throttle control motors. Because the idle control system directly impacts engine stability at low RPM, addressing this fault is important for smooth and safe engine operation.

Quick Reference

  • OBD-II Family: P-Code (Powertrain)
  • Scope: Generic
  • System: Idle Control / Throttle Control
  • Difficulty Level: Moderate
  • Estimated Repair Cost: €50–€250
  • Last Updated: 2025-11-24

Real-World Example / Field Notes

P0505 is extremely common on older Nissan, Honda, and Jeep models using traditional IAC valves. One Honda Accord case involved rough idle and stalling after long drives. The root cause wasn’t the IAC valve itself — the throttle body was heavily carbonized, restricting airflow. After cleaning the throttle body and resetting the ECU idle learn procedure, the idle stabilized immediately. On drive-by-wire vehicles like modern Hyundais and Toyotas, the electronic throttle body motor often becomes sluggish from carbon buildup or internal wear.

Symptoms of P0505

  • Rough idle: Engine shakes or fluctuates when stopped.
  • Stalling at idle: Especially when coming to a stop or when cold.
  • High idle speed: Idle RPM stuck above normal range.
  • Low or unstable RPM: Tachometer dips or surges.
  • Hesitation on takeoff: Idle instability affects throttle response.
  • Check Engine Light: Stored with P0505 and sometimes related throttle codes.

Common Causes of P0505

Most Common Causes

  • Carbon buildup in the throttle body.
  • Faulty or sticking IAC valve (older vehicles).
  • Vacuum leaks from hoses, intake gaskets, or brake booster.
  • Dirty or malfunctioning electronic throttle body (drive-by-wire).
  • Air intake duct leaks or cracked PCV hoses.

Less Common Causes

  • ECU idle learn values corrupted (after battery disconnect).
  • Faulty throttle position sensor or throttle motor (ETCs).
  • Electrical wiring issues to the IAC valve or throttle body.
  • Sticking EGR valve affecting idle airflow (older vehicles).
  • Coolant temperature sensor issues causing incorrect idle targets.

Diagnosis: Step-by-Step Guide

Start by inspecting airflow components and verifying whether idle speed responds to commanded changes.

Tools You’ll Need: OBD-II scanner, multimeter, carb/throttle cleaner, smoke machine (optional), basic hand tools, and a scan tool capable of bi-directional idle control testing.

  1. Check for vacuum leaks. Inspect hoses, intake tubes, and gaskets. A smoke test quickly reveals leaks.
  2. Inspect throttle body condition. If it’s dirty or carbonized, clean it thoroughly.
  3. Test IAC operation (if equipped). Command the IAC valve with a scan tool and look for idle speed change. No response indicates failure.
  4. Check IAC resistance. Compare coil resistance with manufacturer specs.
  5. Inspect the throttle position sensor. Look for dead spots or inconsistent voltage signals.
  6. Evaluate live data. Compare desired idle RPM vs. actual RPM. Large differences indicate idle control issues.
  7. Perform idle relearn procedure. Many vehicles require throttle relearn after battery disconnection or cleaning.
  8. Check for intake or PCV leaks. These cause unmetered air and unstable idle.
  9. Verify coolant temperature readings. Incorrect readings alter ECU idle strategy.
  10. Inspect wiring and connectors. Look for corrosion, loose pins, or broken wires at the IAC valve or throttle body.

Pro Tip: Many drive-by-wire vehicles require a scan tool “throttle body adaptation” after cleaning. Skipping this step often results in continued idle issues and repeated P0505 faults.

Possible Fixes & Repair Costs

  • Clean throttle body: Often fixes idle issues (€0–€40).
  • Replace IAC valve (if equipped): €60–€120 for most vehicles.
  • Repair vacuum leaks: €10–€80 depending on hose or gasket.
  • Perform idle/throttle relearn: Typically free or €30–€60 at a shop.
  • Replace electronic throttle body: €100–€250 depending on model.
  • Replace throttle position sensor: €40–€90.
  • Fix wiring or connector issues: €10–€50.

Always inspect the throttle body and check for vacuum leaks before replacing idle control components — these are by far the most common causes of P0505.

Can I Still Drive With P0505?

It depends on severity. If the idle is only slightly unstable, the vehicle can usually be driven safely. However, if the engine stalls frequently, idles extremely high, or surges unpredictably, driving may be unsafe — especially in traffic or at stoplights. Repair the issue promptly to avoid stalling or poor drivability.

Related Codes

  • P0574 – Cruise Control System Vehicle Speed Too High
  • P0573 – Brake Switch “A” Circuit High
  • P0572 – Brake Switch “A” Circuit Low
  • P0571 – Brake Switch “A” Circuit
  • P0570 – Cruise Control Accelerate Signal
  • P0569 – Cruise Control Coast Signal
  • P0568 – Cruise Control Set Signal
  • P0567 – Cruise Control Resume Signal
  • P0566 – Cruise Control Off Signal
  • P0565 – Cruise Control On Signal

Key Takeaways

  • P0505 indicates a malfunction in the idle control system, typically due to airflow or throttle issues.
  • Most common causes include carbon buildup, vacuum leaks, or a failing IAC valve or throttle body.
  • Driving may be unsafe if the engine stalls or surges significantly.
  • A proper idle relearn is essential after throttle body cleaning or battery disconnection.

FAQ

What causes P0505 most often?

Carbon buildup in the throttle body, vacuum leaks, and failing IAC valves are the most common causes. Newer cars may experience throttle body motor issues.

Can I clean the throttle body to fix P0505?

Yes — cleaning the throttle body often resolves idle issues, especially if carbon buildup is restricting airflow. Be sure to perform a throttle relearn afterward.

Does P0505 mean the IAC valve is bad?

Not always. Many vehicles no longer use an IAC valve. Even on older models, vacuum leaks or throttle body deposits can trigger the code before the valve fails.

Will my car stall with P0505?

It might. Many vehicles exhibit stalling when coming to a stop or when cold. If stalling occurs frequently, avoid driving until repairs are made.

Do I need a throttle relearn after fixing P0505?

Yes. Most modern vehicles require an idle or throttle body relearn procedure after cleaning, battery disconnect, or throttle repairs to restore normal idle behavior.