Quick answer: The P0304 code indicates a cylinder 4 misfire, meaning the engine control module detected that the fourth cylinder is not firing correctly. It affects the engine’s ignition and combustion system. This is a highly urgent issue that requires prompt attention to prevent permanent damage to the catalytic converter.
Seeing the check engine light flash or glow steadily due to a P0304 code can be stressful, especially when accompanied by a rough idle or loss of power. This guide breaks down exactly what a cylinder 4 misfire means for your engine, what symptoms to watch out for, and how to pinpoint the root cause. You will also find practical DIY checks, repair pathways, and realistic cost estimates to help you get back on the road safely.
What Does P0304 Mean?
Your engine relies on precise combustion in each cylinder to generate power smoothly. The “P03” part of the code indicates a general misfire, while the “04” specifically points to cylinder number four. When the engine control module (ECM) monitors the crankshaft speed, it expects a small acceleration boost every time a cylinder fires. If cylinder four fails to contribute its share of power-due to a lack of spark, incorrect fuel delivery, or poor compression-the ECM registers a drop in speed and triggers the P0304 trouble code.
Symptoms
- Rough idling or engine shaking, especially when stopped at a red light.
- A noticeable loss of engine power, hesitation, or jerking during acceleration.
- An illuminated or flashing check engine light on your dashboard.
- A strong smell of unburned fuel coming from the exhaust pipe.
- Decreased fuel economy as the engine struggles to run efficiently.
Possible Causes
Before buying an expensive catalytic converter or fuel injector, start by inspecting the easiest and cheapest components. Jumping straight to the most expensive part is a common and costly mistake.
- Worn, fouled, or improperly gapped spark plug in cylinder 4.
- Failed ignition coil or damaged spark plug wire on cylinder 4.
- Vacuum leak near the intake manifold feeding cylinder 4.
- Clogged or failing fuel injector supplying cylinder 4.
- Mechanical engine issues, such as a burnt valve, worn piston rings, or a blown head gasket causing low compression.
Check These Yourself Before Visiting a Mechanic
- The Coil Swap Test: If your car uses individual coil packs, swap the ignition coil on cylinder 4 with the coil on cylinder 2. Clear the codes and drive. If the code changes to P0302, you have confirmed that the ignition coil is bad and needs replacement.
- Inspect the Spark Plug: Remove the spark plug from cylinder 4 and check its tip. Look for heavy carbon buildup, oil coating, or a cracked ceramic insulator. If it looks worn compared to the others, a simple spark plug replacement may fix the issue.
- Check the Wiring Harness: Visually inspect the electrical connector and wiring harness leading to the cylinder 4 ignition coil and fuel injector. Look for loose connections, frayed wires, or signs of rodent damage.
How to Fix It
- Replace Spark Plugs and Wires: If the spark plug is fouled or has exceeded its service life, replace it along with the ignition wires.
- Replace the Ignition Coil: Swap out the failed cylinder 4 coil pack if the swap test proved it was defective.
- Repair Vacuum Leaks: Inspect the intake manifold and vacuum lines for leaks, replacing damaged gaskets or hoses.
- Service or Replace the Fuel Injector: Clean the fuel system or replace the cylinder 4 fuel injector if it is clogged or electrically dead.
- Address Compression Issues: If compression is low, perform a leak-down test to identify if the issue lies in the valves, piston rings, or head gasket, and perform the necessary internal engine repairs.
Estimated Repair Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Spark Plug Replacement | $10 – $30 | $60 – $120 |
| Ignition Coil Replacement | $40 – $120 | $50 – $100 |
| Fuel Injector Replacement | $80 – $250 | $100 – $200 |
| Intake Manifold Gasket Repair | $20 – $60 | $150 – $300 |
| Engine Compression Repair (Valves/Gaskets) | $150 – $600 | $800 – $2000 |
Note: These are 2026 estimates in US dollars; costs vary by make and model.
Frequently Asked Questions
Can I drive with the P0304 code?
You should avoid driving with this code, especially if the check engine light is flashing. A flashing light indicates an active misfire that is sending raw fuel directly into your exhaust system, which can overheat and destroy your catalytic converter within minutes.
How do I clear the P0304 code?
You can clear the code using an OBD-II scanner connected to your vehicle’s diagnostic port. However, if you do not repair the underlying cause, the engine control module will quickly detect the misfire again and turn the check engine light back on.
How do I find which cylinder is number 4?
Cylinder numbering depends on your engine’s layout. For inline engines, cylinder 4 is usually the fourth one from the front (where the drive belts are). For V-shaped engines, you will need to check your vehicle’s service manual or online diagrams to identify which bank contains cylinder 4.