P0303 Code – Cylinder 3 Misfire (Causes, Symptoms, Fix)

The P0303 trouble code indicates that the engine control unit (ECU) has detected a misfire in cylinder number 3.

A misfire occurs when the air-fuel mixture inside the cylinder fails to burn correctly. This can happen due to ignition problems, fuel delivery issues or mechanical engine faults.

When this happens repeatedly, the ECU records the P0303 diagnostic trouble code (DTC) and usually turns on the Check Engine Light.

Misfires can cause rough engine operation, loss of power and increased emissions.

How Cylinder Misfires Affect the Engine

Modern engines rely on precise combustion timing. If one cylinder stops firing correctly:

• the engine loses balance

• fuel is not burned efficiently

• vibration increases

• emissions rise significantly

If ignored for too long, a misfire can also damage the catalytic converter due to unburned fuel entering the exhaust system.

Common Causes of P0303

Several components may trigger the P0303 code.

Ignition system problems

The ignition system is the most common source of misfires.

Possible issues include:

• faulty spark plug in cylinder 3

• worn ignition coil

• damaged spark plug wire

• poor electrical connection

Fuel delivery issues

If the fuel injector does not supply the correct amount of fuel, combustion may fail.

Examples include:

• clogged fuel injector

• faulty injector wiring

• low fuel pressure

Air intake problems

Incorrect air mixture can also cause misfires.

Possible causes:

• vacuum leaks

• dirty mass airflow sensor

• intake manifold leaks

Mechanical engine problems

More serious issues may also trigger the code.

These include:

• low compression in cylinder 3

• worn piston rings

• burnt valves

• head gasket problems

Symptoms of the P0303 Code

Drivers may notice several symptoms when cylinder 3 misfires.

Common signs include:

• rough idle

• engine vibration

• loss of power

• poor fuel economy

• engine hesitation during acceleration

• flashing Check Engine Light

If the warning light flashes, it indicates a severe misfire that may damage the catalytic converter.

How to Diagnose P0303

Proper diagnosis requires a systematic approach.

Step 1 – Scan the vehicle

Use an OBD2 scanner to confirm the stored code and check for additional codes such as:

• P0300

• P0301

• P0302

• P0304

These may indicate multiple cylinder misfires.

Step 2 – Inspect spark plugs

Remove the spark plug from cylinder 3 and check for:

• oil contamination

• carbon deposits

• worn electrodes

Replacing spark plugs often solves the issue.

Step 3 – Check ignition coils

Swap the ignition coil from cylinder 3 with another cylinder.
If the misfire moves to another cylinder, the coil is defective.

Step 4 – Test the fuel injector

Listen for injector clicking sounds or perform an injector balance test.

Step 5 – Perform a compression test

If ignition and fuel systems are working correctly, a compression test may reveal internal engine problems.

Repair Solutions for P0303

Fixing the problem depends on the root cause.

Common repairs include:

• replacing spark plugs

• replacing ignition coils

• cleaning or replacing fuel injectors

• repairing vacuum leaks

• replacing damaged wiring

• repairing internal engine components

Many P0303 cases are resolved with simple ignition component replacements.

Estimated Repair Cost

Costs depend on the faulty component and vehicle type.

Typical repair ranges:

Early diagnosis can prevent expensive engine damage.

Can You Drive With Code P0303

Driving with a P0303 misfire is not recommended.

Short trips may still be possible, but continued driving can cause:

• catalytic converter damage

• severe engine vibration

• increased fuel consumption

If the Check Engine Light flashes, the vehicle should be inspected immediately.

FAQ – P0303 Code

What cylinder is P0303

P0303 indicates a misfire in cylinder number 3.

Can a bad spark plug cause P0303

Yes. A worn or damaged spark plug is one of the most common causes of this code.

Is P0303 serious

Yes. If ignored, a misfire can damage the catalytic converter and reduce engine performance.

How do I fix code P0303

Start by checking the spark plug and ignition coil in cylinder 3. If the issue remains, inspect fuel injectors and engine compression.