P0303: Cylinder 3 Misfire Detected
P0303 means the engine computer has detected a misfire in cylinder 3, which can shake the engine, reduce power, and damage the catalytic converter if you keep driving it. The most common fix is replacing the spark plug or ignition coil for cylinder 3, which typically costs $150 to $350 at an independent shop.
- 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 P0303 mean?
A misfire means cylinder 3 is not firing properly, so that cylinder is not making normal power. On most gas engines, the cause is usually ignition-related, but it can also come from a bad fuel injector, vacuum leak, compression problem, or wiring fault. The engine computer sets P0303 after it sees crankshaft speed changes that match a misfire on cylinder 3. If the misfire is severe, the check engine light may flash to warn you the catalytic converter can overheat and fail.
Common symptoms
- Check engine light on
- Flashing check engine light
- Rough idle
- Engine shaking or vibrating
- Loss of power
- Bad fuel economy
Causes, fixes and costs
P0303: Cylinder 3 Misfire Detected
General guide for most cars. Your car may differ.
A misfire can overheat and damage the catalytic converter, and a severe one can make the engine run poorly enough to stall. If the check engine light is flashing, the engine is bucking hard, or the car has very little power, stop driving and have it towed.
Most likely causes
- 1
Worn or fouled spark plugHigh
A rough idle, worse acceleration, and a tune-up that is overdue point to a plug problem. If the plug on cylinder 3 is oil-fouled, cracked, or worn, that cylinder may misfire under load or at idle.
Quick check: Remove the cylinder 3 spark plug and inspect the tip for wear, fouling, cracks, or heavy deposits.
- 2
Bad ignition coilHigh
If the misfire comes and goes, or gets worse under acceleration, the coil on cylinder 3 is a common suspect. A coil failure often causes a single-cylinder misfire with a steady rough idle.
Quick check: Swap the cylinder 3 coil with another cylinder and see whether the code follows the coil.
- 3
Fuel injector problemMedium
A clogged or failed injector can cause a dead or weak cylinder, especially if the engine idles rough but ignition parts look good. Fuel-related misfires may be worse on cold starts or under load.
Quick check: Listen for injector clicking and compare cylinder 3 injector operation with the others; a shop can test injector balance.
- 4
Vacuum leak or compression problem on cylinder 3Medium
If the plug and coil test good but the misfire stays on cylinder 3, an air leak near that runner, low compression, burned valve, or head-gasket issue may be the cause. These problems often come with persistent rough idle, low power, or misfire that does not move when parts are swapped.
Quick check: Do a compression test and inspect for vacuum leaks near the intake runner for cylinder 3.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. Basic checks like swapping coils, inspecting spark plugs, and checking for loose connectors are reasonable for many DIYers. You need a spark plug socket, basic hand tools, and ideally an OBD2 scanner; compression or injector testing is better left to a shop.
Show the stepsHide the steps
- Read the code and any companion codes
- Inspect the cylinder 3 coil connector and wiring
- Swap the cylinder 3 coil with another cylinder
- Inspect or replace the cylinder 3 spark plug
- Clear the code and test drive
- If the code returns, check the injector and compression
Estimated repair cost
$120 – $220
Replace spark plug on cylinder 3: parts and labor, often done with one plug or a full set
- Replace ignition coil on cylinder 3$150 – $350
- Clean or replace fuel injector$180 – $450
- Fix vacuum leak$150 – $500
- Engine mechanical repair for low compression$800 – $2,500
Prices vary by vehicle, engine layout, and local labor rates.
Frequently asked questions
Can I drive with P0303?
You may be able to drive gently for a short distance if the engine runs fairly smoothly, but you should get it checked soon. If the check engine light is flashing or the engine is shaking badly, do not keep driving.
Will P0303 clear itself?
Sometimes the code may go away if the misfire was temporary, but the underlying problem often returns. If it comes back, the engine still needs diagnosis.
What is the most common fix for P0303?
The most common fixes are a bad spark plug or ignition coil on cylinder 3. Those are the first parts most shops test because they fail often and are relatively inexpensive.
Can a bad coil cause P0303 and P0300?
Yes. A bad coil can cause a single-cylinder misfire code like P0303, and if the misfire is severe enough the computer may also set the random misfire code P0300.
Will P0303 cause an inspection failure?
Yes, in most states a stored misfire code and a lit check engine light can fail emissions inspection. If the light is flashing, the problem is serious enough that the car should be repaired before 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.