P0304

Cylinder 4 misfire detected.

The computer detected that cylinder 4 was not contributing power consistently. It still does not know what caused that condition.

What should be checked

An insufficient or irregular ignition energy may prevent a stable combustion.

In plain language: The mixture may not be receiving a strong and stable spark.

How to check it: Confirm the engine and follow the authorised ignition sequence without blindly exchanging parts.
Before correcting it: The code does not authorize changing spark plugs, coils, cables or modules. The correction is decided only after locating and confirming the defect.
Related components: This path may involve spark plugs, ignition coils, high-tension leads if fitted, connectors, or wiring. None has been confirmed.
When to stop: High voltage and a running engine require proper tools and training.

The quantity, pressure or fuel control may not match the engine's calculated demand.

In plain language: The engine may be receiving too much or too little fuel, but it still needs to be measured.

How to check it: Reviewing correction data and performing only the pressure or control tests indicated for the exact system.
Before correcting it: Do not open lines or replace injectors or pumps without a confirmatory measurement. The correction is decided only after locating and confirming the defect.
Related components: This path may involve fuel injectors, the pump, regulator, filter, lines, or their electrical controls. None has been confirmed.
When to stop: Fuel pressure, leakage, strong smell or smoke require immediate professional attention.

Unmeasured air, a restriction or incorrect flow control may cause the calculated values to not match.

In plain language: Air may enter an unexpected place or not circulate the amount the computer calculates.

How to check it: Visualizing accessible connections and then using the leakage or flow method authorised for the engine.
Before correcting it: Do not use smoke, pressure or disassemble the intake without knowing its limits. The correction is decided only after locating and confirming the defect.
Related components: This path may involve the air filter, ducts, clamps, hoses, gaskets, throttle body, or airflow control components. None has been confirmed.
When to stop: Stop the review if it requires pressurizing, disassembling intake or working near hot or moving parts.

A mechanical difference can reduce the cylinder input even if air, fuel and spark are present.

In plain language: The cylinder may not be closing, compressing or moving like the others.

How to check it: Comparative testing only with the exact engine method and limits.
Before correcting it: The code does not authorize the disassembly of the backrest, valves or distribution. The correction is decided only after locating and confirming the defect.
Related components: This path may involve valves, the valvetrain, pistons, rings, cylinders, the head gasket, or valve timing components. None has been confirmed.
When to stop: Consult a professional before an internal test or any engine dismantling.

Choose the make and open its official portal to find the manual, diagrams, cylinder arrangement, and bulletins for the exact vehicle.

Availability, registration, and cost depend on the manufacturer. The link does not grant permission to copy the manual.

Understand which emissions-related service information manufacturers must make available for purchase.

Explains regulated access in the United States; it does not contain the vehicle-specific diagram or make the manual free.

Audi A3 Quick Reference Specification Book — Audi of America / archivo NHTSA. It supports the punctual definition of the code; It does not identify a cause, a part or a repair.
Frequently Asked Questions About On-Board Diagnostics — U.S. Environmental Protection Agency. Supports the safety difference between a fixed breakdown lamp and a flashing lamp; It doesn't diagnose the cause.