Hyundai Elantra
Misfire/rough idle? Start with trims + cylinder evidence. Many Elantra “misfires” are mixture or EVAP-related, not coils.
Quick triage
Capture
- STFT/LTFT at idle + 2,000 rpm
- Misfire counters per cylinder (if available)
- Fuel pressure (if PID available)
- EVAP purge command vs idle stability
Fast interpretation
- Rough after refuel → purge valve stuck open / vent control issues.
- Single-cylinder repeats → ignition/injector/mechanical, not “random”.
- All-cylinder random with lean trims → air leak / fuel delivery.
- P0420 → prove stable upstream first.
Common complaints
- Intermittent idle shake: check trims and EVAP purge behaviour; a purge valve bleeding vapour can mimic ignition faults.
- Hesitation / flat spot: trims-first, then air measurement plausibility, then ignition.
- P0300: use counters to find whether it’s actually one cylinder repeating.
Typical OBD2 codes
P0300
Random misfire: use evidence, not guesses.
P0171
Lean: split idle vs load trims to pick the right path.
P0420
Cat efficiency: confirm mixture + misfire health first.
P0442
EVAP small leak: pressure cap vs purge/vent logic.
High-confidence check: If STFT swings positive at idle but settles near zero at 2,000 rpm, you’re chasing an air leak/PCV path — not a “bad sensor”.
Trust note: These profiles are designed to narrow possibilities. Confirm with test data (trims, misfire counters, pressure/smoke tests, voltage checks) before buying parts.