P0171 can fail an MOT in the UK, usually because it keeps the engine management light on and can also create emissions problems if the engine is genuinely running lean. The exact test outcome depends on how the fault presents on the day, but as a practical rule you should treat P0171 as an MOT-risk code and fix it before booking.
Yes, P0171 is an MOT-risk code in the UK because it often keeps the warning light on and can also affect emissions if the lean condition is genuine. The best assumption is that it should be diagnosed and fixed before the test rather than cleared and ignored.
This guide uses common UK MOT outcomes, fault-code behaviour, and practical lean-mixture diagnosis to explain why P0171 matters before a test. It is intended as educational guidance, not a replacement for live fuel-trim analysis or inspection. For how we explain diagnostics and review tools, see our editorial approach, customer reviews, and scanner recommendations.
P0171 means the ECU is adding fuel because it believes bank 1 is too lean. That often points to air leaks, MAF issues, fuel-delivery weakness, or another mixture-balance fault. If the ECU cannot maintain clean combustion and emissions control, the warning light and the emissions readings can both work against you.
Yes, sometimes. That is why P0171 catches people out before MOT. The car may still feel usable while the ECU quietly adds fuel to compensate. But even if it drives acceptably, the light and emissions risk are enough to make it worth fixing before the test.
Use your fault code and vehicle details to see likely causes, MOT implications, and realistic repair context before spending money.
Get My AI Diagnostic Report — £1.59 →Disclaimer: Informational guidance only. Live MOT outcomes depend on the exact vehicle, the fault state at test time, and current DVSA testing rules.