OBD-II Code P1056 — What It Means & How to Fix It

What Does This Error Mean?

P1056 means the Idle Speed Performance fault has occurred. The PCM attempted to control idle speed but the result was outside the expected range. Unlike a simple idle control circuit code, P1056 means the idle control hardware is responding but the engine idle speed is not settling correctly. You may notice a rough, surging, or unstable idle — the engine may hunt up and down in RPM rather than holding a steady speed.

Affected Models

Common in some Chrysler, Dodge, and Jeep vehicles with idle speed monitoring
Common in Dodge Neon, Stratus, and Sebring with 2.0L and 2.4L four-cylinder engines
Also seen on some Honda and Acura vehicles with IAC-related performance monitoring
May appear on any vehicle with electronic idle air control and PCM idle speed monitoring

Common Causes

Dirty or stuck Idle Air Control (IAC) valve not responding correctly to PCM commands
Large vacuum or air leak bypassing the throttle body and causing excess uncontrolled idle air
Carbon buildup on the throttle plate creating inconsistent airflow at idle
Failed throttle position sensor causing the PCM to misread closed throttle position
Incorrect base idle speed — throttle stop screw adjusted incorrectly on adjustable throttle bodies

How to Fix It

Check for vacuum leaks at all hoses connected to the intake manifold, throttle body, and brake booster. Even a small leak causes unstable idle.

Spray throttle body cleaner or soapy water around vacuum connections while idling. A change in RPM confirms a leak location.
Clean the throttle body and throttle plate. Heavy carbon deposits on the throttle plate cause the idle to fluctuate as air flow sticks and releases.

Remove the air intake duct and spray throttle body cleaner on the plate and bore. Wipe clean with a lint-free cloth.
Remove and clean the Idle Air Control valve. Use IAC-specific or throttle body cleaner. Spray through both ports.

If the IAC plunger does not extend and retract smoothly after cleaning, replace the valve.
Check the throttle position sensor. At idle with the throttle fully closed, TPS should read approximately 0.45 to 0.55V.

An incorrect TPS idle voltage causes the PCM to misidentify the throttle position and set an incorrect idle target.
After cleaning, clear the code and perform an idle relearn if required by your vehicle. Allow the engine to idle for 10 minutes to recalibrate.

On Honda and Acura vehicles, the ECM idle relearn requires specific ignition cycle procedures — consult the service manual.

When to Call a Professional

IAC valve cleaning or replacement costs $100 to $300. Throttle body cleaning is $100 to $250. Vacuum leak diagnosis and repair is $100 to $400. Base idle adjustment is a quick shop service — typically $50 to $100.

Frequently Asked Questions

Can a dirty throttle body cause P1056?

Yes — heavy carbon buildup on the throttle plate is one of the most common causes. Carbon deposits change the airflow characteristics at closed throttle. The PCM tries to compensate but the idle speed oscillates rather than stabilizing.

What is idle relearn and why do I need to do it?

After throttle body cleaning or IAC replacement, the PCM may have old idle position data stored. An idle relearn allows the PCM to reestablish the correct closed-throttle baseline. Skipping this step can result in a persistently unstable or high idle.

Is P1056 dangerous to drive with?

It depends on severity. Mild idle hunting is annoying but safe. If the idle drops so low the engine stalls at traffic lights, it becomes a safety issue. Fix it soon to avoid stalling in traffic.