Fault codeP0300

SEVERE

Random/Multiple Cylinder Misfire Detected

What this code means

One or more cylinders misfire irregularly. The engine may shake and lose power, and the catalytic converter can be damaged if the problem persists.

P0300 is a generic code: it means the same thing on every make. P0300 belongs to family P03xx — Ignition and misfire. Its page explains how to read a code and groups all its codes by part.

Combustion (misfire detection)

The control unit watches the speed of the crankshaft at every combustion. When a cylinder fails to fire, the crankshaft slows slightly: the control unit counts these misfires cylinder by cylinder.

Symptoms

Engine shaking at idle, jerking under acceleration, loss of power, sometimes stalling. The check engine light may flash: that means unburnt fuel is reaching the catalytic converter and destroying it — stop driving. The code is “random” because the misfires hit several cylinders or move between them, which points to a shared cause rather than one bad spark plug.

Common causes

Worn or badly gapped spark plugs, tired leads or coils on several cylinders
Lean mixture: intake air leak, dirty airflow sensor (see P0171)
Insufficient fuel pressure: clogged filter, weak pump, dirty injectors
EGR valve stuck open, diluting the mixture at idle
Less often: low compression (valves, rings), timing out of position (P0016), head gasket

How to diagnose it

First note the conditions: idle only, cold only, or under load? A cold misfire that disappears once warm points to ignition or a leak that closes with expansion; a misfire under load points to fuel delivery or a weak coil.

If codes P0301 to P0308 are also present, they name the affected cylinders. Start with the spark plugs: their appearance says a lot — whitish, the mixture is lean; black and dry, it is rich; oily, oil is getting past. In live data, long-term fuel trim above +10% confirms a lean mixture. With no lead, a compression test settles ignition versus mechanical.

What to do

Have it checked promptly. A misfire sends unburnt fuel into the exhaust and destroys the catalytic converter within a few dozen kilometres. If the light is flashing, stop driving.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

A set of spark plugs costs €30 to €120, a coil €40 to €150. An air leak is often fixed with a hose costing a few euros. If the cause is mechanical — compression, timing — expect several hundred to well over a thousand euros.

How to read this code

PPowertrain
0standardised code
3Ignition and misfire
00P0300