Fault codeP0305

SEVERE

Cylinder 5 Misfire Detected

What this code means

Cylinder 5 is not burning the air-fuel mixture properly. Likely causes: worn spark plug, faulty coil or clogged injector.

P0305 is a generic code: it means the same thing on every make. P0305 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

The engine shakes, especially at idle, and lacks power; the idle is uneven and consumption rises. Unlike P0300, the control unit has identified cylinder 5: the fault comes from that cylinder or from what feeds it. If the light is flashing, stop: unburnt fuel destroys the catalytic converter.

Common causes

Cylinder 5 spark plug worn, fouled or cracked
Failing ignition coil or plug lead on this cylinder
Dirty injector, spraying badly or leaking
Low compression on this cylinder: valve no longer sealing, worn rings
Head gasket or intake gasket leaking at this cylinder

How to diagnose it

The simplest method is swapping: exchange the cylinder 5 coil with a neighbour's, clear the code and drive. If the misfire follows the coil, it is the cause; otherwise do the same with the spark plug, then the injector.

If the misfire stays on cylinder 5 despite these swaps, the cause is mechanical: a compression test, then a leak-down test, locate the problem. On an older engine, an oily plug on this one cylinder is a sign of ring wear.

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 spark plug costs €8 to €30, a coil €40 to €150, an injector €60 to €300 depending on the engine. A compression fault is another matter: from a few hundred euros for a cylinder head to far more for an engine.

How to read this code

PPowertrain
0standardised code
3Ignition and misfire
05P0305