System Too Lean (Bank 1)

What it means

The engine computer is adding more fuel than normal to make up for too little fuel or too much air, and has reached its maximum correction on the bank containing cylinder 1. The most common cause is unmetered air entering the engine.

Related codes: P0174P0300P0101P0087

Symptoms4

• Check engine light on.
• Rough or slightly shaky idle.
• Weak acceleration or hesitation on the accelerator.
• Occasional hard starting, especially after sitting for a long time.

Possible causes6

• Vacuum leak from hoses or the intake manifold gasket.
• Dirty mass airflow (MAF) sensor under-reading the incoming air.
• Low fuel pressure from the pump, a clogged filter or a faulty regulator.
• Air leak at the PCV valve or its hose.
• Dirty fuel injectors.
• Exhaust leak ahead of the upstream oxygen sensor.

How to diagnose5

1. Check short- and long-term fuel trims with a scan tool; high trims at idle that drop at higher RPM usually mean a vacuum leak.
2. Have a smoke test done on the intake manifold and vacuum hoses.
3. Compare the MAF reading with the expected value for the engine size.
4. Measure fuel pressure at idle and under load.
5. If P0174 is also stored, the cause is shared by both banks rather than on one side.

How to fix5

1. Replace the leaking hose or gasket.
2. Clean the MAF sensor with a dedicated cleaner, and replace it if readings stay wrong.
3. Replace the fuel filter, pump or regulator based on the pressure test.
4. Clean the injectors or replace any faulty one.
5. Clear the codes and confirm the fuel trims return to normal.

The fix is often a vacuum hose or a MAF cleaning at low cost; it rises when the fuel pump or injectors are at fault.

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

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