P0303 fault code: cylinder 3 misfire

What does it mean?

P0303 means the misfires the engine computer has counted in cylinder 3 have gone past the set limit. A misfire is when the fuel-air mixture in a cylinder doesn't ignite or burns only partly; on most cars the engine computer can tell this from momentary slowdowns in the crankshaft's rotation.

The code pointing to a single cylinder suggests the cause is usually in a part that belongs to that cylinder. On many petrol engines each cylinder has its own spark plug, its own ignition coil and its own injector; the problem may be in one of these, or in the cylinder's compression. If other cylinders have misfire codes too, a common cause such as fuel pressure or an air leak becomes more likely.

You may notice the engine shaking at idle, misfiring and hesitating when you press the accelerator, or losing power. On some cars the misfire may be more noticeable when the engine is cold or at a specific rpm.

How serious is it?

Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.

Common general causes

• Spark plug wear or fouling (on petrol engines)
• This cylinder's ignition coil
• Clogged or leaking injector on this cylinder
• Wiring, connector or connection problems
• Air leak in the intake passage to the cylinder
• Low compression (valves, piston rings, gaskets)

The list shows general causes; a workshop finds the actual cause on your car with a check.

Related codes

• P0300Random Misfire in Multiple Cylinders
• P0301Cylinder 1 Misfire
• P0302Cylinder 2 Misfire
• P0304Cylinder 4 Misfire