smart fortwo
Short trips and small batteries make voltage a frequent root-cause. Start with cranking/charging plausibility, then use trims and repeatable conditions to find the real fault.
Quick triage (5 minutes)
What to capture
- Does it do lots of short journeys? Any recent battery/jump-start history?
- Cranking voltage (ideally): does it dip very low during start?
- Charging voltage at idle with lights/demist on
- Fuel trims at idle and at 2,000 rpm
- When symptom happens: cold start, hot restart, after refuel, in rain
What it usually means
- Random warnings / weird behaviour: low voltage or poor charging stability.
- Rough idle / stalling: trims-first (air leak vs fueling), EVAP purge patterns, or ignition.
- After-refuel stumble: EVAP purge behaviour — confirm timing and trims.
Usually is / Usually isn’t
Usually is
- Battery/charging plausibility issue (especially if short trips)
- Vacuum leak or mixture control issue that shows in fuel trims
- EVAP purge pattern after refuelling
Usually isn’t
- Multiple unrelated failures at once
- “Cat failure” without proving misfire/mixture first
- Random sensor replacement without a plausibility check
Common codes and what to do with them
- P0171: compare trims idle vs 2,000 rpm; smoke test before parts.
- P0300 / misfire: confirm with counters; check ignition basics and mixture first.
- P0456: EVAP small leak — smoke test is the reliable path.
Confirmatory tests
- Voltage under crank: low dips can create cascading symptoms.
- Charging under load: headlights + rear demist on; watch stability.
- Trims comparison: idle vs 2,000 rpm to separate leak vs fueling.
- Smoke test: fastest way to settle “air leak” and EVAP debates.
Trust note: If symptoms are intermittent and “all over the place”, start with voltage and grounds. Fixing low voltage often makes other codes disappear.