P0308 fault code: Cylinder 8 Misfire Detected High severity

P0308 means “Cylinder 8 Misfire Detected” — a ignition system & misfire fault in the powertrain. A flashing light means unburnt fuel is reaching the catalytic converter. Typical UK repair cost: £30–£350 depending on the underlying cause.
HighSeverity
P03xxIgnition System & Misfire
£30–350Typical UK repair cost
MOT fail riskif the warning light stays on

What does P0308 mean?

P0308 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P03xx block, which covers ignition system & misfire. The stored description is “Cylinder 8 Misfire Detected”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed, which is why a short diagnosis pays for itself before replacing components.

Symptoms you may notice

  • Shaking or vibration at idle
  • Flashing engine management light under acceleration
  • Loss of power and poor fuel economy
  • Popping or spitting noises from the exhaust

Most common causes

  1. Worn spark plugs or failed ignition coil (the most common cause in UK cars)
  2. Faulty HT leads on older engines
  3. Low compression in one cylinder (valve or head gasket issues)
  4. Contaminated or stale fuel, blocked injector

How to diagnose P0308 yourself

  1. Read the code to identify which cylinder is misfiring (P0301 = cylinder 1, P0302 = cylinder 2, and so on)
  2. Swap the ignition coil with a neighbouring cylinder and re-scan — if the misfire moves, replace the coil
  3. Inspect and gap (or replace) the spark plugs
  4. If the misfire stays put, run a compression test on that cylinder
Tools that read and clear P0308

A basic code reader confirms the fault; a full-system scanner shows the live data you need for the steps above.

Will P0308 fail an MOT?

A flashing or steady engine management light is a Major defect — an automatic fail. Misfires almost always push emissions beyond the MOT limits too.

Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.

Typical UK repair costs

OutcomeTypical cost (parts + labour)
DIY: read code + basic checks£0 (scanner assumed)
Independent garage diagnosis£30–£60
Typical repair range for this fault£30–£350
Main dealer equivalent£45–£630

Costs are indicative UK ranges for independent garages in 2026. Main dealers typically charge 40–80% more.

Diagnose the symptom, then price the repair

Car diagnostic test cost UK (2026): scan vs actual diagnosis£40–£100 is typical for an independent first assessment — but a code read is not the same as finding the cause. Engine management light flashing and car juddering: stop or drive?A flashing EML plus shaking usually means an active misfire — treat it differently from a steady light. Catalytic converter replacement cost UK (2026): don’t replace it from P0420 aloneMainstream fitted prices are often £250–£900, but integrated and type-approved parts can run higher.
P0300Random/Multiple Cylinder Misfire Detected P0301Cylinder 1 Misfire Detected P0302Cylinder 2 Misfire Detected P0303Cylinder 3 Misfire Detected P0304Cylinder 4 Misfire Detected P0305Cylinder 5 Misfire Detected P0306Cylinder 6 Misfire Detected P0307Cylinder 7 Misfire Detected

P0308 — quick answers

Is it safe to drive with P0308?

A flashing light means unburnt fuel is reaching the catalytic converter. Stop driving as soon as it is safe — continuing can turn a £30 spark plug job into a £1,000+ catalytic converter replacement.

Will P0308 clear itself?

If the underlying fault stops occurring, most ECUs extinguish the light after around 40 healthy warm-up cycles — but the code stays stored as history. Fix the cause, clear it with a scanner, then confirm it stays away over a full drive cycle.

How much does it cost to fix P0308 in the UK?

Typically £30–£350 at an independent garage depending on which cause you confirm. Diagnosing it yourself with a scanner avoids a £30–£60 diagnostic fee and stops parts being replaced on guesswork.

Will P0308 fail my MOT?

A flashing or steady engine management light is a Major defect — an automatic fail. Misfires almost always push emissions beyond the MOT limits too.

Code definitions follow the SAE J2012 standard. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.