⚠ P0300 HIGH SEVERITY

P0300: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0300 — The engine is misfiring on multiple or random cylinders."

What OBD Mechanic tells you

"This is a random or multiple-cylinder misfire — the engine's firing unevenly across more than one cylinder rather than pinning it to just one. Because it's spread out, the cause is usually something shared: worn spark plugs or coils on a petrol engine, a vacuum leak, low fuel pressure or bad fuel. If you also see cylinder-specific codes like P0301-P0308 alongside it, those tell you exactly where to start. Start with the cheap wear items — plugs and coils — before chasing anything deeper. Petrol engines misfire on ignition; diesels don't have plugs, so on a diesel look more at injectors, fuel and compression."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

Worn or fouled spark plugs (petrol) — cheapest and most common
Failing ignition coil(s) or leads (petrol)
Vacuum leak or low fuel pressure affecting several cylinders at once
Bad or contaminated fuel, or a weak fuel pump
Injector problems (especially on diesels) or, worst case, mechanical wear / low compression

usually cheap depends pricey

Suggested checks

Check for any cylinder-specific codes (P0301-P0308) — they point straight to the culprit
On petrol, inspect and if due replace the spark plugs — cheap and often the fix
Swap a coil to a different cylinder and see if the misfire follows it
Consider recent fuel — a bad tankful can do this; sometimes it clears once burned through
If it persists, check fuel pressure and look for vacuum leaks

Which dials to watch for P0300

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

RPM (live graph)

A misfire shows up as the idle bouncing or dipping. Watching RPM at idle lets you feel how rough it's running and whether it's getting worse.

Short fuel trim

Helps separate a fuel/air cause from an ignition one. Big swings here point at a mixture problem feeding the misfire rather than a dead coil.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Live simulation on the readings our free demo drive supports. This is how the app’s dials move.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0300

This is a random or multiple-cylinder misfire — the engine's firing unevenly across more than one cylinder rather than pinning it to just one. Because it's spread out, the cause is usually something shared: worn spark plugs or coils on a petrol engine, a vacuum leak, low fuel pressure or bad fuel. If you also see cylinder-specific codes like P0301-P0308 alongside it, those tell you exactly where to start. Start with the cheap wear items — plugs and coils — before chasing anything deeper. Petrol engines misfire on ignition; diesels don't have plugs, so on a diesel look more at injectors, fuel and compression.

P0300: quick questions

Can I still drive it?

Go easy. A misfire dumps raw unburnt fuel into the exhaust, which can quickly wreck an expensive catalytic converter — especially if the engine light is flashing, which means it's misfiring badly right now. If the light's flashing, ease off the power and stop driving as soon as it's safe — damage is happening while it flashes. With a steady light, drive gently and get it looked at soon; a bad misfire can also leave you stranded.

How much will it cost to fix?

If it's plugs and coils on a petrol, you might spend £30-60 on plugs or £30-80 per coil, and many do it themselves. Bad fuel can cost nothing but a tank of fresh. The expensive end is injectors or a compression problem, but those are less common — start with the cheap stuff.

Will it pass the MOT?

A steady engine light means an automatic MOT fail now under the emissions-light rule. Beyond that, a misfiring engine will often fail the emissions test too and can run rough enough to be noticed. Get it fixed before the test.