Worn or fouled spark plug on cylinder 4; Faulty ignition coil or coil connector on cylinder 4; Faulty or blocked fuel injector on cylinder 4; Vacuum or intake leak near cylinder 4, such as a cracked hose or leaking intake gasket; Damaged wiring or poor connection to the coil or injector on cylinder 4; Low compression on cylinder 4 from valve, piston ring or head gasket problems; Fuel pressure problem or sensor fault affecting mixture or timing, such as airflow, crankshaft or camshaft sensor.

P0304

Cylinder 4 Misfire Detected.

Generic
Powertrain
Generic code
Medium severity

How this code is built

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

What it means

P0304 means the engine control unit has detected misfires on cylinder 4 (generic SAE J2012 definition). It monitors crankshaft speed fluctuations and sees that cylinder 4 is not contributing its share of power, with the misfire count above the set threshold. The cause can be ignition, fuel, air or mechanical, and the exact layout depends on the model and engine.

Symptoms

Rough or uneven idle
Engine hesitation, stumbling or loss of power under acceleration
Engine management light on, flashing if the misfire is severe
Smell of unburnt fuel from the exhaust
Engine shaking or juddering, especially at idle or under load
Poor fuel economy or difficulty starting

Possible causes

Worn or fouled spark plug on cylinder 4likely
Faulty ignition coil or coil connector on cylinder 4likely
Faulty or blocked fuel injector on cylinder 4likely
Vacuum or intake leak near cylinder 4, such as a cracked hose or leaking intake gasketpossible
Damaged wiring or poor connection to the coil or injector on cylinder 4possible
Low compression on cylinder 4 from valve, piston ring or head gasket problemspossible
Fuel pressure problem or sensor fault affecting mixture or timing, such as airflow, crankshaft or camshaft sensorrare

How to diagnose

Read all stored codes and freeze frame data to see load, rpm, temperature and whether other cylinders are also misfiring
Check live misfire counters to confirm the misfire is only on cylinder 4 and note when it occurs
Inspect the coil, connector, wiring and injector plug for cylinder 4 for damage, corrosion, oil or heat damage
Remove and inspect the cylinder 4 spark plug for wear, fouling, wet fuel, oil or damage, and check the gap where applicable
Swap the cylinder 4 coil with another cylinder and see if the misfire follows, then clear codes and retest
Check for vacuum leaks and intake leaks, and review fuel trims for signs of a lean condition
Test injector operation and resistance on cylinder 4, and check fuel pressure against the specification for the engine
Perform a compression or leak-down test on cylinder 4 if ignition and fuel are ruled out

How to fix

Replace the spark plugs, ideally as a full set, to the manufacturer's specification
Replace the faulty ignition coil on cylinder 4
Clean or replace the faulty fuel injector on cylinder 4
Repair or replace damaged wiring, connectors or leaking intake hoses and gaskets
Correct fuel pressure or sensor faults found during testing
Carry out the necessary engine repair if compression is low, such as valve or head gasket work

Can I keep driving?

Medium severity

Light use is possible if the engine runs reasonably smoothly, but get it fixed soon because a continuing misfire can overheat and damage the catalytic converter. If the warning light is flashing or the engine runs badly, stop driving and have it checked.

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

This code for other makes

P0304AudiWorn 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)