System Too Lean at Idle (Bank 1)

What it means

The engine computer has found that bank 1 runs lean at idle, and it is adding as much fuel as it can without correcting the mixture. Because airflow at idle is small, any unmetered air has a big effect, so the most common cause is a vacuum leak.

Related codes: P2189P2177P0171P0507P0101

Symptoms4

• Check engine light on.
• Unstable idle or shaking at a stop.
• Sometimes a hissing sound from the intake manifold area.
• Occasional stalling at a stop.

Possible causes6

• Vacuum leak from hoses or the intake manifold gasket.
• Leak at the PCV valve or its hose.
• Air leak from the brake booster hose.
• Dirty mass airflow (MAF) sensor under-reading the incoming air.
• Low fuel pressure or dirty injectors.
• Exhaust leak ahead of the upstream oxygen sensor.

How to diagnose5

1. Watch the fuel trims; high values at idle that drop at higher RPM usually mean a vacuum leak.
2. Listen for hissing and inspect the vacuum hoses and brake booster hose.
3. Have a smoke test done on the intake manifold and hoses.
4. Check the PCV valve and its hose.
5. If P2189 is also stored, the cause is shared by both banks rather than on one side.

How to fix5

1. Replace the leaking vacuum hose or gasket.
2. Replace the PCV valve or its hose if cracked.
3. Clean the MAF sensor with a dedicated cleaner.
4. Repair any exhaust leak ahead of the oxygen sensor.
5. Clear the codes and confirm the trims return to normal at idle.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The fix is often a vacuum hose or gasket at low cost; it rises when the leak is at the intake manifold gasket, which takes more labor.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions