P0300 logs when the ECU detects misfires happening across more than one cylinder. Unlike P0301–P0308 (which point to a specific cylinder), P0300 means the problem is general — coil power supply, fuel pressure, a big vacuum leak or contaminated fuel.
Top UK causes (in order)
1. Worn spark plugs (especially if past their service interval) 2. Multiple coil packs failing together (heat-related) 3. Big vacuum leak (split intake or PCV hose) 4. Low fuel pressure — failing fuel pump or blocked filter 5. Bad batch of fuel 6. Carbon-fouled direct-injection inlet valves (VW TSI, BMW N20)
What to fix first
Replace all spark plugs as a set if they're over 30,000 miles old (or 60,000 for iridium). It's £80–£160 and fixes about 40% of P0300s on cars over 80,000 miles.
UK costs
- Spark plug set + fitting: £80–£160 - Coil packs (each): £80–£200 fitted - Fuel filter: £80–£140 fitted - Walnut blast inlet clean: £350–£600 (BMW/VW direct injection)
Can I drive with P0300? — Short journeys at light throttle, yes. A persistent misfire kills the cat (£600+) within weeks.
Will it fail the MOT? — Yes if the EML is on. Also, the emissions sniff test usually fails with active misfires.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.