System Too Lean (Bank 2)

What it means

The engine computer is adding more fuel than normal on bank 2 to make up for extra air or too little fuel, and has reached its maximum correction. It appears on engines with two banks, and if P0171 appears with it, the cause is usually shared, such as a large vacuum leak, the airflow sensor or weak fuel delivery.

Related codes: P0171P0173P0151P0101P0087P0300

Symptoms4

• Check engine light on.
• Shaky or uneven idle.
• Hesitation and loss of power on acceleration.
• Occasional hard starting.

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, filter or regulator.
• Air leak at the PCV valve or its hose.
• Clogged injector on bank 2.
• Leak in the bank 2 exhaust manifold ahead of the oxygen sensor.

How to diagnose5

1. Compare fuel trims between banks; high trims on both suggest a shared cause, on bank 2 alone a cause on that side.
2. Note whether the correction is higher at idle and drops with engine speed, the typical pattern of a vacuum leak.
3. Have a smoke test done on the intake manifold and vacuum hoses.
4. Measure fuel pressure at idle and under load against the manufacturer specification.
5. Compare the MAF reading with the expected value for the engine size.

How to fix5

1. Replace the leaking hose or gasket.
2. Clean the MAF sensor, and replace it if readings stay wrong.
3. Replace the fuel filter, pump or regulator based on the pressure test.
4. Clean or replace clogged bank 2 injectors, and repair any exhaust leak ahead of the sensor.
5. Clear the codes and confirm fuel trims on both banks return to normal.

The fix is often a vacuum hose or MAF cleaning at low cost; it rises when the fuel pump, intake manifold gasket 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

Frequently asked questions