CASE 004

P0171 System Too Lean (Bank 1)

intermediate 20 min Petrol

Vehicle

ModelTraining Estate 1.6
Engine1.6 16V
Mileage145,000 km

Customer complaint

Rough idle, occasional hesitation and a check engine light. Fuel consumption has increased slightly.

History

Air filter replaced recently. No other work.

Fault codes

P0171 System too lean, bank 1

Live data

Long-term fuel trim, bank 1Strongly positive
MAF reading at idleLower than expected
O2 sensor upstreamSwitching lean

Measurements

Intake smoke testLeak found at the intake manifold gasket

Diagnostic case

Next step — Reasoning.

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Confirmed cause

Unmetered air entering after the MAF sensor through a leaking intake manifold gasket.

Reasoning

Positive fuel trims mean the ECU is adding fuel to correct a lean condition. When the MAF reads low at idle and a smoke test finds a leak downstream of the sensor, the diagnosis is unmetered air — the engine receives more air than the ECU measures.

Recommended repair

Replace the intake manifold gasket and any cracked hoses.
Clear the code and reset fuel trims.
Road test and confirm trims return within the expected band.

Related tools

OBD-II Code Lookup Live OBD PID Data Repair Cost Calculator

Related OBD codes

P0171

All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.