P0171

What the Code Means

P0171 flags a lean air-fuel mixture on Bank 1. The DME sees the air-to-fuel ratio shifted toward excess air. On N55 engines the usual causes are unmetered air (including through the PCV system), a dirty mass air flow sensor (MAF), and low fuel pressure. The control unit compensates with fuel trim, but once the mismatch crosses the threshold, the code sets.

How It Works Normally

The DME continuously balances the mixture off the oxygen sensor signals. Small deviations get absorbed by fuel trim within tolerance; going past tolerance means the mechanical or fuel system isn't compensating and needs diagnosis.

Symptoms

Warning Lights

• Check Engine Light on

Engine and Drivability

• rough idle
• hesitation under acceleration
• reduced power
• possible misfires

Diagnosis and Repair

Step 1. Check for Unmetered Air

Run a smoke test on the intake system — on the N55 this is the first and most likely check.

Step 2. Check the PCV System

Inspect the PCV valve — on the N55 it's integrated into the valve cover. If it fails, the valve cover gets replaced as a unit, OEM part number 11127588412.

Step 3. MAF Sensor

Clean or replace the mass air flow sensor, OEM part number 13627588475. A common diagnostic mistake is skipping this step when the sensing element is clearly dirty.

Step 4. Fuel System

Check fuel pressure and injector condition.

Secondary Damage

Running lean for a long time leads to overheating and pre-ignition, which can damage pistons and valves. Excess oxygen in the exhaust also kills the catalytic converter. Don't put off diagnosis: the chain from overheating to piston damage develops over prolonged driving.