Misfire
Shaking, hesitation, flashing engine light. Diagnose the cylinder and cause before replacing random parts.
HIGH RISK
Common symptoms
- Engine shakes under load or at idle
- Flashing CEL (urgent)
- Fuel smell / poor MPG
- Catalytic converter overheating risk
Cost expectations
DIY: £10–£150. Garage: £120–£450+ depending on diagnosis.
Note: prices vary by car and region. Use this as a planning range.
Most likely causes (ranked)
- Spark plugs worn/fouled — Most common and cheap to fix.
- Coil pack / ignition lead fault — Often heat-related or load-related misfires.
- Injector issue — Clogged/leaking injector causes lean/rich misfire.
- Air leak / vacuum leak — Skews mixture on one bank/cylinder.
- Compression problem — Valve/piston/head gasket issues (less common, more serious).
Quick checks (fast, low-cost)
- Scan codes and note cylinder (P0301 etc.).
- Swap coil with another cylinder; see if misfire follows.
- Inspect/replace plugs (correct heat range and gap).
- Check fuel trims; big positive trims suggest air leak.
- Do a compression test if misfire persists after ignition/fuel checks.
Recommended tools
These tools make diagnosis faster and stop you buying random parts.
Related fixes
If your symptoms don’t match perfectly, check these next.
What to do next
- If CEL is flashing, avoid hard driving.
- Fix ignition items first (fast/cheap).
- Only move to compression testing once basics are ruled out.
AutoSolveHub helps you narrow possibilities. Always confirm critical repairs with proper testing and safe procedures.