Is the idle relearn procedure really necessary?

On many vehicles, yes. After cleaning the throttle body, some PCMs need to relearn the idle parameters. Skipping it can cause the idle to stay low even after a clean throttle body.

Can P0506 cause the car to stall in drive?

Yes. At low idle, adding load from the transmission in drive can pull RPM below the stall threshold. This is especially common when the AC is running.

Is this different from P0505?

P0505 is a general idle control system fault. P0506 specifically means idle is too low. Both often share the same causes and fixes.

Codes › P0506

P0506: The engine is idling too low

Idle Air Control System RPM Lower Than Expected

Moderate severityCan you keep driving?

Drive to a shop soon. An idle that is too low causes stalling at stops, which can be a safety hazard in traffic.

What it means

P0506 means the engine's actual idle speed is consistently lower than what the computer is commanding. The computer is asking for a target idle RPM, but the engine cannot reach it. The most common reason is a restriction in airflow: carbon on the throttle body or IAC valve, or a vacuum leak that is confusing the system into overcorrecting. It can also happen when the idle load suddenly increases, such as when the AC compressor kicks in, and the engine cannot compensate quickly enough.

Common causes

Carbon buildup on throttle bodyThe most common cause. Deposits restrict airflow at idle. Often fixes with a simple cleaning.
Faulty or carbon-clogged IAC valveRestricts the bypass air that maintains idle speed under load.
Vacuum leakCauses the idle to hunt and the PCM may overcorrect, resulting in a code.
Clogged fuel injectors or low fuel pressureNot enough fuel at idle can pull the RPM down below target.

What it costs

Throttle body cleaning$80 to $150
IAC valve cleaning or replacement$50 to $300
Vacuum leak repair$80 to $250

Symptoms

Low idle, engine may run at 400 to 500 RPM when it should be near 700 to 800
Stalls at stops, especially when AC is on
Rough idle
Check engine light on

What to do next

Start with throttle body cleaning. It is cheap, takes less than an hour, and fixes P0506 more often than any other single repair. After cleaning, do the idle relearn procedure for your specific vehicle; many require it before the PCM will hold idle correctly again. Look up your vehicle's relearn steps before you start.

Lewis shows the commanded idle RPM target versus the actual idle RPM in real time, which tells you immediately how far off the engine is and helps confirm whether cleaning fixed the gap. Download Lewis.

Related codes

FAQ

Is the idle relearn procedure really necessary?: On many vehicles, yes. After cleaning the throttle body, some PCMs need to relearn the idle parameters. Skipping it can cause the idle to stay low even after a clean throttle body.
Can P0506 cause the car to stall in drive?: Yes. At low idle, adding load from the transmission in drive can pull RPM below the stall threshold. This is especially common when the AC is running.
Is this different from P0505?: P0505 is a general idle control system fault. P0506 specifically means idle is too low. Both often share the same causes and fixes.