P0304: Cylinder 4 Misfire Detected
P0304 means the engine computer has detected a misfire on cylinder 4. The most common fix is replacing a bad spark plug or ignition coil, which typically costs about $150 to $450 at an independent shop, depending on the vehicle and access.
- Severity
- High
- Can I drive?
- Drive with caution
- DIY?
- Possible with some experience
- Repair cost range
- $120 – $2,500
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What does P0304 mean?
A misfire happens when cylinder 4 does not burn its air-fuel mixture properly. That can be caused by weak spark, a fuel delivery problem, low compression, or an air leak affecting that cylinder. When one cylinder misfires, the engine may shake, run rough, and set the check engine light. If the misfire is severe enough, the light may flash to warn that engine or catalytic converter damage can happen quickly.
Common symptoms
- Rough idle
- Engine shaking
- Check engine light on
- Check engine light flashing
- Hesitation on acceleration
- Loss of power
Causes, fixes and costs
P0304: Cylinder 4 Misfire Detected
General guide for most cars. Your car may differ.
A misfire can damage the catalytic converter and make the engine run poorly. If the check engine light is flashing, the engine is shaking badly, or the car is losing power, stop driving and have it towed.
Most likely causes
- 1
Worn or fouled spark plugHigh
A rough idle, hesitation, or a misfire that gets worse under load often points to a plug that cannot fire cylinder 4 consistently.
Quick check: Remove and inspect the cylinder 4 spark plug for heavy wear, oil, carbon buildup, or a cracked insulator.
- 2
Bad ignition coil or coil bootHigh
If the misfire is random but stays on cylinder 4, especially with shaking and a flashing light, the coil is a top suspect.
Quick check: Swap the cylinder 4 coil with another cylinder and see whether the code moves to that cylinder.
- 3
Fuel injector problemMedium
A misfire with a cylinder-specific fuel trim issue, fuel smell, or no change after ignition parts are checked can point to injector trouble.
Quick check: Listen for injector clicking and compare cylinder 4 spark plug condition to the others; a scan tool can also help compare cylinder balance.
- 4
Low compression or internal engine wearLow
If cylinder 4 keeps misfiring after plugs, coil, and injector checks, and the engine has constant roughness or poor power, mechanical damage is possible.
Quick check: Run a compression test or leak-down test on cylinder 4 and compare it with the other cylinders.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. Basic DIY checks usually include inspecting and swapping the spark plug and ignition coil, which needs simple hand tools and the correct replacement parts. If the problem does not move with those parts, a shop should test fuel delivery and engine compression.
Show the stepsHide the steps
- Scan for codes and note any other cylinder or fuel codes.
- Inspect the cylinder 4 spark plug and coil for damage, oil, or corrosion.
- Swap the cylinder 4 coil with another cylinder and clear the code.
- Drive or idle the car again and see whether the misfire follows the coil.
- If needed, inspect or replace the spark plug.
- If the code stays on cylinder 4, have the injector and compression tested.
Estimated repair cost
$150 – $300
Spark plug replacement for one cylinder: parts and labor for one plug, often more if plugs are replaced as a set
- Ignition coil replacement for one cylinder$180 – $450
- Fuel injector replacement for one cylinder$250 – $650
- Compression or leak-down diagnosis$120 – $250
- Engine mechanical repair$500 – $2,500
Prices vary by vehicle, engine layout, parts quality, and regional labor rates.
Frequently asked questions
Can I drive with P0304?
You may be able to drive a short distance if the engine is only slightly rough, but it is better to get it checked soon. If the check engine light is flashing or the engine is shaking hard, stop driving.
Will P0304 clear itself?
Sometimes the code may go away if the problem is temporary, but the fault often comes back. If the cause is still there, the misfire will return under the same conditions.
Is P0304 always a bad spark plug?
No. A bad spark plug is common, but a coil, injector, vacuum leak, low compression, or wiring problem can also cause it. The cylinder-specific code helps narrow the diagnosis, but it does not identify the exact part by itself.
Can P0304 damage my catalytic converter?
Yes. A persistent misfire can send unburned fuel into the exhaust and overheat the catalytic converter. That is why a flashing check engine light should be treated as urgent.
Can I fix P0304 myself?
Many people can replace a spark plug or swap an ignition coil at home. If the code stays on cylinder 4 after that, a shop is usually needed for injector and compression testing.
Related codes
Related problems
This guide covers most cars and is general advice, not an inspection. Prices are typical US independent-shop ranges and vary by region, car and shop. If your brakes, steering or a warning light feel unsafe, stop driving and contact a mechanic.