Updated April 2026 · AI-Diagnostics-Pro · 5 min read
The check engine light appears but the car feels completely normal. Here's what's likely happening and what you should do.
P0456/P0457 — Loose fuel cap. Tighten or replace the cap.
P0420/P0430 — Catalytic converter degrading. Car drives fine but will fail MOT.
P0128 — Thermostat fault. Uses more fuel but drives normally.
P0171/P0174 — Running lean. Often no symptoms until it worsens.
Yes. Any illuminated check engine light is an automatic MOT failure — regardless of how the car is driving. Diagnose and fix it before booking your test.