Drivable — worth looking into
Check engine came on after refuelling
You filled up, drove a few kilometres, and the light came on. The car drives exactly as before.
There is no danger, but the symptom will not go away by itself: it is the early stage of something that gets pricier.
What to look at in the live data
- The code itself: the fuel cap and the vapour system have their own numbers, and they are harmless for driving.
- Fuel trims: a sharp move to plus right after refuelling points at the fuel, not the cap.
- The freeze frame: if the fault was recorded at speed on a warm engine, that is the leak test — it runs exactly in those conditions.
Common causes, cheapest first
- The fuel cap is not closed to the click, or its seal has hardened — the most common cause, and it costs nothing.
- The vapour system tests itself after refuelling: it sees a leak where you would notice nothing.
- Bad fuel: the mixture changes, the trims move, and the unit records a mixture fault rather than a cap one.
Codes that show up with this
P0455Evaporative emission system leak detected (gross leak)P0442Evaporative emission system leak detected (small leak)P0457Evaporative leak — fuel cap left looseP0171System too lean (bank 1) — air leak or not enough fuel
These parameters in the reference
Short term fuel trim B1Long term fuel trim B1Ethanol fuel percentageCommanded equivalence ratioO2 sensor B1S2 voltageShort term fuel trim B2Long term fuel trim B2Distance with MIL onTime run with MIL onDistance since codes clearedTime since codes clearedWarm-ups since codes cleared
Lamps that may be on
Do it step by step
P0171 is not a random string but an address. We take it apart, sign by sign.Reading a fault code: the letter and four digits
A symptom is not a diagnosis. The same sign comes from a dozen different causes — the readings are what tell them apart, not the feeling.
How the app helps
CarScan shows exactly these readings live, remembers the highest and the lowest value of the trip, and puts them into a report you can hand to a mechanic.