smart forfour
Use the same fundamentals: voltage stability, trims-first mixture diagnosis, and confirmatory tests. The forfour adds practicality, but the logic stays the same.
Quick triage
What to capture
- Cranking and charging voltage (especially if symptoms are intermittent)
- Fuel trims at idle and 2,000 rpm
- Misfire counters (if present) and which cylinder dominates
- When it happens: after refuel, during rain, hot restart, motorway load
What it usually means
- Rough idle: mixture/air leak vs ignition — trims + counters decide the direction.
- Stall after filling up: EVAP purge/refuel pattern.
- Random warnings: voltage stability and grounds first.
Common codes
- P0171: trims point you to air leak vs fueling; smoke test is the fast answer.
- P0300: confirm counters; prove coil/plug/mixture before replacing parts.
- P0456: small EVAP leak — smoke test is the reliable route.
Confirmatory tests
- Smoke test for intake/EVAP leaks.
- Voltage checks under crank and under electrical load.
- Trims comparison idle vs 2,000 rpm for leak vs fueling direction.
Trust note: If you fix a confirmed voltage issue, re-scan afterwards. Many secondary codes disappear once supply is stable.