Worn or faulty spark plug on cylinder 3; Failed ignition coil or coil connector on cylinder 3; Faulty or dirty fuel injector on cylinder 3; Vacuum or intake leak near cylinder 3 (split hose, intake gasket); Low cylinder compression from valve, piston ring or head gasket problems; Damaged wiring or connector to the injector or coil; Faulty crankshaft or camshaft position sensor giving false misfire detection.

P0303

Cylinder 3 Misfire Detected.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
3: AreaIgnition system or misfire
03: FaultThe specific fault in that area

What it means

P0303 means the engine control unit has detected misfiring on cylinder 3. It sets when crankshaft speed sensing shows that cylinder is not contributing its share of power, either intermittently or constantly, beyond a set threshold. The code only says which cylinder is affected, not why, so the cause could be ignition, fuel, air or mechanical.

Symptoms

Rough or uneven idle
Engine shaking or juddering, especially under load
Loss of power and hesitation on acceleration
Flashing or constant engine warning light
Smell of unburnt fuel from the exhaust
Hard starting or occasional stalling

Possible causes

Worn or faulty spark plug on cylinder 3likely
Failed ignition coil or coil connector on cylinder 3likely
Faulty or dirty fuel injector on cylinder 3possible
Vacuum or intake leak near cylinder 3 (split hose, intake gasket)possible
Low cylinder compression from valve, piston ring or head gasket problemspossible
Damaged wiring or connector to the injector or coilrare
Faulty crankshaft or camshaft position sensor giving false misfire detectionrare

How to diagnose

Read all stored codes and freeze frame data to see engine load, speed and temperature when the misfire set, and check for other misfire or fuel trim codes.
Visually inspect the coil connector, injector connector and wiring on cylinder 3 for damage, corrosion or looseness.
Check for intake leaks by inspecting hoses and joints, and review fuel trim data for signs of a lean condition.
Remove the cylinder 3 spark plug and compare its condition and gap with the others.
Swap the cylinder 3 coil with another cylinder, clear the code, and see if the misfire follows the coil.
Swap or test the cylinder 3 injector and check its electrical operation and spray or flow.
Perform a compression test or leak-down test on cylinder 3 if ignition and fuel are ruled out.
Check fuel pressure and sensor data against the manufacturer's specification for the specific engine, as procedures vary by model.

How to fix

Replace worn spark plugs, ideally as a full set to the correct specification.
Replace the faulty ignition coil on cylinder 3.
Clean or replace the faulty fuel injector, depending on test results and the engine type.
Repair vacuum leaks by replacing damaged hoses or intake gaskets.
Repair damaged wiring or connectors.
Carry out the necessary engine mechanical repair if compression is low, such as valve or head gasket work.

Can I keep driving?

High severity

Avoid driving if the warning light is flashing or the engine is running very roughly, as unburnt fuel can overheat and damage the catalytic converter. If it is only a mild, steady misfire, drive gently and short distances to get it repaired promptly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

This code for other makes

P0303AudiWorn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems)