P0171 fault code: System Too Lean
Last updated · OBDCode UK editorial
A lean mixture with a steady light can often be driven short distances with care. Stop if the engine surges badly, overheats, or the light starts flashing.
Can I DIY?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.
Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.
Never idle in a closed garage to read codes — an open door is not enough. CO is odourless and can kill. Run diagnostics in open air. Headache or nausea with exhaust smell: get fresh air and stop.
What does P0171 mean?
P0171 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P01xx block, which covers fuel and air metering. The stored description is “System Too Lean”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
How to diagnose P0171 yourself
- Confirm P0171 and fuel trims: Scan while stationary in open air. Record P0171 as current or pending, freeze-frame, short-term and long-term fuel trim on bank 1, and any MAF or oxygen-sensor companions. Stop if: The light is flashing, the engine will not idle, or you would have to run it in a closed garage.
Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim. - Inspect intake vacuum hoses in the bay: With the engine cold and the bonnet open, look along vacuum and crankcase hoses for cracks, splits, or pipes off their stubs. Do not remove the inlet manifold or any EGR valve. Stop if: Hoses sit where you would need a lift, the engine is hot, or an electric fan may start. Keep clear of the fan shroud.
- Watch live oxygen and trim data: On live data, watch bank-1 oxygen or AFR readings and fuel trims at idle, then at a steady 2,000 rpm in open air. Do not spray flammable cleaners into the intake. Stop if: The engine revs on its own, you smell petrol, or you cannot do this test outdoors.
Will P0171 fail an MOT?
An illuminated engine management light is a Major defect at MOT (automatic fail) until the lean fault is repaired and the lamp stays off. A mixture that is too lean can also push the exhaust emissions test out of limits.
Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.
What the MOT data says
No MOT certificate item corresponds to this code; MOT outcome data cannot be shown.
Related guides
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P0171 — quick answers
Is it safe to drive with P0171?
Drive with care. A lean mixture with a steady light can often be driven short distances with care. Stop if the engine surges badly, overheats, or the light starts flashing.
Can I fix P0171 myself?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.
Will P0171 fail my MOT?
An illuminated engine management light is a Major defect at MOT (automatic fail) until the lean fault is repaired and the lamp stays off. A mixture that is too lean can also push the exhaust emissions test out of limits.
Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.