P0171

System Too Lean (Bank 1)

Symptoms

• •Rough idle
• •Hesitation
• •Reduced economy
• •Check engine light

Likely causes

• •Vacuum/intake leak
• •Dirty MAF sensor
• •Weak fuel pump
• •Clogged injectors

Can I drive?

Diagnose soon to avoid running issues.

Estimated repair cost (Malaysia)

RM200–RM800 (approximate)

Diagnostic guide (for mechanics)

Work through these in order — cheapest and safest checks first, before replacing expensive parts.

1. 1. Read fuel trims

   Confirm the lean condition and note whether it is worse at idle (suggests a vacuum leak) or at higher load (suggests fuel delivery or a dirty MAF).

   Test: Live data short- and long-term fuel trim, bank 1.

   Expected: Lean means positive trims well above +10%.

2. 2. Inspect for vacuum/intake leaks

   Check the intake boot, PCV hoses, brake booster line, and gaskets. A leak after the MAF is a common cause.

   Test: Smoke test the intake, or spray carb-cleaner near joints and watch idle.

   Expected: No idle change / no smoke escaping — i.e. no leak.

3. 3. Inspect the MAF sensor

   A dirty or failing MAF under-reports airflow, so the ECU adds too little fuel. Clean with MAF cleaner and compare readings.

   Test: Live data MAF g/s at idle vs at a steady RPM.

   Expected: Plausible airflow that rises smoothly with RPM (no flat/erratic signal).

4. 4. Check fuel delivery

   If air metering and leaks are good, test fuel pressure and consider clogged injectors or a weak pump.

   Test: Fuel-pressure gauge at idle and under load.

   Expected: Pressure holds to the vehicle's spec under load.

Tools: OBD-II scan tool · Smoke machine or carb cleaner · MAF cleaner · Fuel-pressure gauge

When to escalate: If trims stay lean after leaks, MAF, and fuel pressure check out, refer for deeper fuel-system or exhaust (pre-O2) leak diagnosis.

This is general information, not a diagnosis. AutoMalaya OBD helps explain your car’s actual scan data; a qualified mechanic may still need to inspect the vehicle.

Related fault codes

See sample report