P0303 — Cylinder 3 Misfire Detected

Quick answer

• What the code means: The Powertrain Control Module (PCM) has detected a misfire in cylinder 3.
• Severity: Get it checked out soon, but you can typically drive home cautiously.
• Most common cause: A failing spark plug, ignition coil, or fuel injector for cylinder 3.

What the P0303 code means

The official OBD-II definition for P0303 is "Cylinder 3 Misfire Detected." In simpler terms, this means that the engine's computer (the PCM) has identified that cylinder number 3 is not contributing its fair share of power to the engine. This usually happens when the air-fuel mixture isn't igniting correctly or completely within that cylinder. The PCM closely monitors the rotational speed of the crankshaft. If it detects a momentary slowdown that doesn't align with normal engine operation, it interprets this as a misfire in one of the cylinders. When it consistently detects this slowdown corresponding to cylinder 3's firing order, it sets the P0303 code.

Common symptoms

• Check Engine Light illuminated or flashing (a flashing light indicates a severe misfire that could damage the catalytic converter).
• Rough idling or stumbling.
• Reduced engine power and acceleration.
• Decreased fuel economy.
• Engine hesitation or stumbling under load.
• A strong smell of raw fuel from the exhaust (due to unburnt fuel).
• Engine knocking or pinging sounds.

Common causes

1. Faulty Spark Plug (Cylinder 3): Worn, fouled, or incorrectly gapped spark plugs are a very common cause of misfires. A weak or absent spark won't ignite the air-fuel mixture.
2. Defective Ignition Coil (Cylinder 3): Most modern engines use coil-on-plug ignition systems, meaning each cylinder has its own ignition coil. If the coil for cylinder 3 fails, it won't send enough voltage to the spark plug.
3. Clogged or Failing Fuel Injector (Cylinder 3): A fuel injector that is clogged, stuck open, or not opening at all will either deliver too little or too much fuel to cylinder 3, leading to an incorrect air-fuel mixture that won't ignite properly.
4. Vacuum Leaks: Leaks in the intake manifold gasket or vacuum lines can lean out the air-fuel mixture in certain cylinders, leading to misfires. While less specific to a single cylinder, a localized leak could affect cylinder 3.
5. Low Compression (Cylinder 3): Worn piston rings, damaged valves, a blown head gasket, or a cracked cylinder head can all lead to a loss of compression in cylinder 3. Without adequate compression, the air-fuel mixture cannot be ignited effectively.
6. Wiring Issues: Damaged wiring to the spark plug, ignition coil, or fuel injector for cylinder 3 can prevent proper operation.
7. PCM (Powertrain Control Module) Issues: While rare, a faulty PCM could incorrectly detect a misfire or fail to control the ignition or fuel delivery for cylinder 3. This is usually diagnosed after all other possibilities have been ruled out.

How a technician diagnoses it

1. Verify the code and check for other codes: The technician will connect an OBD-II scan tool to confirm the P0303 code and check for any other related codes that could provide additional clues.
2. Check freeze-frame data: The scan tool will be used to view freeze-frame data, which shows the engine conditions (engine speed, load, temperature, etc.) at the moment the P0303 code was set. This information can help replicate the conditions under which the misfire occurs.
3. Inspect ignition components: The spark plug and ignition coil for cylinder 3 will be visually inspected for damage, cracks, or corrosion. The spark plug will be removed to check its condition and gap. If the engine has a traditional distributor, the cap, rotor, and wires leading to cylinder 3 will be inspected.
4. Perform a "swap" test: If suspecting a spark plug or ignition coil, the technician may swap the cylinder 3 component with a known good component from another cylinder (e.g., cylinder 1). If the misfire code then moves to the cylinder it was moved to (e.g., P0301), it confirms the swapped component is faulty.
5. Test fuel injector operation: The technician might use a stethoscope to listen to the fuel injector for cylinder 3 to ensure it's clicking, indicating it