How much does it cost to fix P0303?

DIY repair typically costs $15–$250 for parts. Professional repair runs $150–$1200 including parts and labor.

P0303

Powertrain

Moderate

Cylinder 3 Misfire Detected

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The P0303 trouble code indicates that your vehicle's engine control module (ECM) has detected a misfire specifically in cylinder 3. A misfire occurs when the air-fuel mixture in a cylinder fails to ignite properly or doesn't ignite at all. The ECM monitors engine speed and crankshaft position hundreds of times per second, and when it detects that cylinder 3 isn't contributing its expected power output during the combustion cycle, it logs this code and illuminates the Check Engine Light. If the misfire is severe or continuous, the Check Engine Light may flash to warn you of potential catalytic converter damage from unburned fuel.

Cylinder 3 misfires matter because they affect your engine's performance, fuel economy, and emissions. When one cylinder isn't firing properly, the remaining cylinders must work harder to compensate, leading to reduced power and acceleration. More importantly, unburned fuel from the misfiring cylinder gets pushed into the exhaust system, where it can overheat and damage the expensive catalytic converter. The raw fuel also increases harmful emissions and causes your vehicle to fail emissions testing. You may notice your engine running rough, shaking at idle, or hesitating during acceleration.

Addressing a P0303 code promptly is important to prevent further damage and more expensive repairs. In many cases, the fix is straightforward and affordable—often just a worn spark plug or faulty ignition coil specific to cylinder 3. Start with the simplest and most common causes first: inspect and replace the spark plug, then check the ignition coil if needed. If these components are in good condition, you may need to investigate fuel injectors, vacuum leaks, or compression issues. While you can often continue driving with a misfire for a short period, it's best to diagnose and repair the issue within 1-2 weeks to avoid catalytic converter damage and prevent the problem from worsening.

Severity

Moderate — Address SoonWithin 1-2 Weeks

Symptoms

•Engine runs rough or shakes, especially at idle
•Noticeable loss of power during acceleration
•Check Engine Light is illuminated and may flash during acceleration
•Increased fuel consumption and poor gas mileage
•Strong smell of unburned gasoline from exhaust
•Engine hesitation or stumbling when accelerating

Likely Causes

Worn or fouled spark plug in cylinder 335%

Spark plugs wear out over time and can become fouled with carbon deposits, preventing proper ignition. This is the most common cause of single-cylinder misfires and is relatively inexpensive to fix.

Faulty ignition coil or coil pack for cylinder 330%

Ignition coils can fail due to heat, vibration, or age, preventing adequate spark delivery to the cylinder. Coil failure is especially common in coil-on-plug systems found in modern vehicles.

Vacuum leak affecting cylinder 3 or low compression15%

A vacuum leak near cylinder 3 or worn piston rings, valves, or head gasket can cause low compression, preventing proper combustion. This often requires more extensive diagnosis and repair.

Clogged or faulty fuel injector for cylinder 312%

A dirty or electrically faulty fuel injector can prevent proper fuel delivery to cylinder 3. This is more common in vehicles with high mileage or those that have used low-quality fuel.

Damaged spark plug wire or boot (if applicable)8%

In vehicles with traditional spark plug wires, damaged insulation or corroded connections can allow spark energy to leak before reaching the plug, causing a misfire.