The check-engine light says almost nothing. The code behind it says almost everything. The big three:
P0420 — Catalyst efficiency
Usually a tired catalytic converter or O2 sensor. Not urgent, but fails emissions tests. Garage quote range: wide — knowing the code protects you.
P0300 — Random misfire
Spark plugs, coils, or fuel delivery. Feels like shuddering. Address soon — misfires can cook the catalyst (see P0420).
P0171 — Running lean
Vacuum leak or dirty MAF sensor most often. Often a €20 fix (cleaning spray) misdiagnosed as bigger work.
Read them yourself
A €25 Bluetooth reader + free app shows code, freeze-frame data and lets you clear after repair. The TopScan AI goes further: plain-language cause + severity.