Mitsubishi Lancer
Lancer faults are usually solvable fast if you separate mixture control from ignition and avoid sensor guessing. Start with trims and plausibility checks.
Quick triage (5 minutes)
What to capture
- Codes + freeze-frame
- STFT/LTFT at idle and 2,000 rpm
- MAF g/s (or MAP kPa) at idle + light cruise
- O2 sensor switching (if narrowband) or equivalence ratio (if wideband)
- Misfire counters per cylinder (if available)
What it usually means
- Lean trims at idle only - intake/PCV air leak or MAF bias.
- Lean under load - fuel delivery limitation or airflow under-reporting.
- Misfire with normal trims - ignition, compression, or injector balance.
- Rough idle after refuel - EVAP purge behaviour before coils.
Common Lancer complaints (starting point)
- Intermittent misfire / hesitation: confirm trims and misfire counters before replacing parts. A mixture issue can masquerade as ignition.
- Stall/rough idle when warm: check for small vacuum leaks, purge influence, and throttle body/idle control contamination (verify via data, not assumption).
- P0420 with otherwise "fine" running: treat it as a system check - prove the engine is not misfiring/over-fuelling and the upstream sensor data is sane before condemning the catalyst.
Usually is / usually is not
Usually is
- Air leak or MAF bias causing trim drift
- Weak ignition on one cylinder (confirmed by misfire counters)
- EVAP purge flow influencing idle stability
- O2 sensor ageing creating slow/biased feedback
Usually is not
- ECU failure
- Random sensor replacement fixing it long-term
- Cat failure without upstream causes (for P0420)
- "Bad fuel" as a diagnosis (unless proven)
Typical OBD2 codes
P0171
Lean Bank 1: trims-first plan that stops guessing.
P0300
Random misfire: narrow it with counters + mixture logic.
P0420
Catalyst efficiency: upstream issues that masquerade as a bad cat.
P0456
EVAP small leak: smoke-test logic and quick sanity checks.
Confirmatory checks (fast)
- Smoke test if trims point lean at idle.
- MAF sanity: compare expected airflow vs measured; a biased MAF creates a believable but wrong story.
- Ignition proof: use misfire counters and plug inspection; move a suspect coil to see if the misfire follows (only after mixture is stable).
- EVAP isolation: temporarily clamp purge line or command purge off (where safe) to see if idle stabilises.
Trust note: The goal is to narrow possibilities. Confirm with test data before buying parts.