P0171 ("system too lean, bank 1") tells you the ECU is pushing fuel trims to their maximum to compensate for unmetered air entering the engine. It's almost always a vacuum leak — not a fuel problem.
Top UK causes
1. **Split intake hose or perished gasket** — most common 2. **Failed PCV valve** (BMW, Mini, Audi especially) — £80–£250 3. **Dirty MAF sensor** — clean for £15 of MAF cleaner 4. **Leaking injector seal** (Ford EcoBoost, VW TSI) 5. **Blocked fuel filter** — usually shows P0171 with low-power complaints
DIY checks
Spray carb cleaner around intake hoses and the throttle body with the engine idling. If the idle changes, you've found the leak. Costs about £6 for a can of carb cleaner.
UK costs
- Intake hose replacement: £80–£180 - PCV valve: £80–£250 fitted - MAF clean: £20–£60 - Fuel filter (diesel): £80–£140
Can I drive with P0171? — Yes, but you'll be losing 5–10% MPG and risking cat damage long-term. Fix within a month.
Will it fail the MOT? — Only if the EML is on at MOT time. The fault itself doesn't test emissions directly.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.