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P0304

Worn 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).

  • Audi
  • 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
04
FaultThe specific fault in that area

What it means

P0304 means the engine control unit has detected misfires on cylinder 4. It sets when crankshaft speed monitoring shows the cylinder is not contributing its share of power, either randomly or repeatedly. On an Audi the cause is most often ignition-related, but fuel, air and mechanical faults can also trigger it.

Symptoms

  • Engine management light on, possibly flashing under load
  • Rough idle and noticeable shaking
  • Hesitation or loss of power when accelerating
  • Hard or prolonged starting, especially when cold
  • Smell of unburnt fuel from the exhaust
  • Increased fuel consumption

Possible causes

  • Worn or faulty spark plug on cylinder 4likely
  • Failed ignition coil on cylinder 4likely
  • Faulty or blocked fuel injector on cylinder 4possible
  • Vacuum or intake leak affecting cylinder 4, such as a split hose or leaking intake gasketpossible
  • Low or weak fuel pressure or a fuel delivery faultpossible
  • Mechanical fault on cylinder 4, such as low compression, valve problem or leaking head gasketrare
  • Faulty crankshaft or camshaft sensor, or wiring and connector fault to the coil or injectorrare

How to diagnose

  1. Read all stored codes and freeze frame data to see load, engine speed and temperature when the fault set, and check for other misfire or fuel trim codes
  2. Inspect the cylinder 4 coil connector, wiring and the plug well for oil, moisture or damage
  3. Remove the cylinder 4 spark plug and check its condition and gap against others; replace the set if worn
  4. Swap the cylinder 4 coil with another cylinder, clear the codes, and see if the misfire follows the coil
  5. Check live misfire counters and fuel trims to separate a fixed cylinder 4 fault from a general lean or random misfire
  6. Smoke test the intake and check hoses for vacuum leaks
  7. Test the injector on cylinder 4 for correct operation and resistance, and check fuel pressure to the manufacturer's specification
  8. Carry out a compression or leak-down test on cylinder 4 if ignition and fuel checks find nothing; procedure and values depend on the engine

How to fix

  • Replace the spark plugs as a set with the correct type for the engine
  • Replace the faulty ignition coil
  • Repair or replace the faulty or blocked injector on cylinder 4
  • Repair vacuum or intake leaks and replace damaged hoses or gaskets
  • Repair wiring or connector faults, or replace a faulty crankshaft or camshaft sensor
  • Carry out the required engine repair if a mechanical fault is confirmed

Can I keep driving?

High severity

Avoid driving if the engine management light is flashing, as raw fuel can overheat and damage the catalytic converter. If the light is steady and the engine runs smoothly enough, drive gently and short distances only until it is repaired.

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

This code for other makes

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