P0171 (System Too Lean) – Vauxhall EcoTEC patterns

What it usually means on Vauxhall

P0171 is almost always a fuel-trim story. On EcoTEC petrol engines, a small unmetered-air leak or a MAF that reads low can drive trims up without obvious drivability.

Fast checks that save time

• Look at STFT/LTFT at idle vs 2500rpm. Big trims at idle that improve off-idle often points to a vacuum/PCV leak.
• Compare MAF g/s at idle and under a steady 2500rpm to expected values for the engine size.
• Smoke test intake, PCV hoses and brake booster line. Don’t forget the oil cap/dipstick seal on some engines.

Common misdiagnosis

Replacing O2 sensors without proving mixture error. The sensor is usually reporting correctly.