P0304
Cylinder 4 Misfire Detected
Get Your FREE AI Diagnostic Report
Enter your vehicle for a personalized diagnosis for code P0304
OBD2 trouble code P0304 indicates that your vehicle's engine control module (ECM) has detected a misfire in cylinder 4. A misfire occurs when the air-fuel mixture in a cylinder fails to ignite properly or doesn't ignite at all, disrupting the smooth operation of your engine. The ECM continuously monitors engine performance through crankshaft position sensors, and when it detects irregular rotation patterns consistent with a misfire in cylinder 4 specifically, it triggers this diagnostic trouble code and illuminates the check engine light.
This code matters because continued driving with a cylinder misfire can cause serious problems beyond just poor performance. Unburned fuel can damage your catalytic converter—an expensive emissions component that can cost $1,000 or more to replace. The misfire also reduces fuel efficiency, increases harmful emissions, and puts additional stress on other engine components. While P0304 is typically not an emergency requiring immediate roadside assistance, it should be diagnosed and repaired within a week to prevent cascading damage and more expensive repairs down the line.
The good news is that P0304 is often caused by relatively simple issues like a worn spark plug or failing ignition coil, both of which are affordable repairs that many DIY mechanics can handle. Start by inspecting the spark plug and ignition coil for cylinder 4, as these are the most likely culprits. If those components check out, you may need to investigate fuel injector problems, compression issues, or vacuum leaks. Getting a proper diagnosis from a mechanic or performing systematic troubleshooting yourself will help you address the root cause efficiently and get your engine running smoothly again.
Severity
Symptoms
- •Check Engine Light illuminated
- •Rough idle or engine vibration
- •Loss of power during acceleration
- •Decreased fuel economy
- •Engine hesitation or stumbling
- •Strong smell of unburned fuel from exhaust
Likely Causes
Spark plugs wear out over time and can fail to properly ignite the air-fuel mixture. This is the most common cause of cylinder-specific misfires and is relatively easy to diagnose and fix.
Ignition coils can fail due to age, heat, or electrical stress, preventing adequate spark delivery to the cylinder. Modern coil-on-plug systems make this a common failure point.
A clogged or failed fuel injector can prevent proper fuel delivery to the cylinder, causing a lean misfire condition that triggers the code.
Worn piston rings, damaged valves, or a blown head gasket can reduce compression, making it difficult for the cylinder to fire properly. This typically requires more extensive repairs.
Air leaks in the intake manifold or vacuum lines can cause an improper air-fuel mixture, leading to misfires. While less common for single-cylinder codes, localized gasket failure can affect individual cylinders.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace spark plug in cylinder 4 (most common fix)
- Replace ignition coil for cylinder 4
- Clean or replace fuel injector for cylinder 4
- Perform compression test and repair valve/piston issues if low compression found
- Inspect and replace intake manifold gasket or vacuum lines if leaks detected
Shop Parts
Videos
P0300 Explained - Random/Multiple Cylinder Misfire Detected (Simple Fix)
FIXD
How to Fix a Random Engine Misfire in Your Car (Code P0300)
Scotty Kilmer
Spark Plug Replacement DIY (the ULTIMATE Guide)
ChrisFix