01 P0049 Turbocharger or Supercharger Turbine Overspeed • 02 P0100 - MAF Sensor Circuit Insufficient Activity • 03 Fuel Pressure Regulator Issues: Symptoms and Repairs • 04 Is it cheaper to run a generator on gas or propane? • 05 Engine Thirst: Identifying the Sounds of a Car in Need of O… • 06 Advantages & Challenges of Different Piston Head Designs "C… • 07 P0045, P0046, P0047, and P0048: Turbo or Supercharger Boost… • 08 Experts Warn That A Small Everyday Driving Mistake Could En… • 09 Idle Dilemma: Drive or Neutral at Traffic Lights? • 10 This Car Can Literally Park Itself In Spaces You Wouldn’t E… •

DTC codes / P03xx · Ignition system and misfire / P0303

P0303Cylinder 3 Misfire Detected

P0303 means the engine computer has detected a misfire on cylinder 3, which is the third cylinder in the engine's firing order. The powertrain control module monitors crankshaft speed fluctuations and flags this code when cylinder 3 fails to contribute its expected power stroke, often counting misfires over a specific number of engine cycles.

SystemPowertrain
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • The check engine light may be steady or flashing, with a flashing light indicating a severe misfire that can damage the catalytic converter.
  • The engine may idle roughly, shake, or feel like it is stumbling, especially when cold or under light load.
  • You may notice a noticeable loss of power or hesitation during acceleration, as the engine is effectively running on one fewer cylinder.
  • A strong raw fuel smell from the exhaust may be present because unburned fuel is being pushed through the engine.
  • Fuel economy often drops noticeably, and the vehicle may fail an emissions inspection.
  • Often no symptoms besides the warning light, particularly during light-load cruising where the misfire may be intermittent.

Common causes, most likely first

  1. Worn or fouled spark plug on cylinder 3, which is the most common cause and often the easiest to check.
  2. Failed ignition coil or spark plug wire/boot dedicated to cylinder 3, leading to a weak or no-spark condition.
  3. Faulty fuel injector for cylinder 3, such as a clogged nozzle, internal short, or wiring issue preventing proper fuel delivery.
  4. Vacuum leak near cylinder 3's intake runner, such as a cracked intake manifold gasket or deteriorated vacuum hose, causing a lean misfire.
  5. Low compression on cylinder 3 due to worn piston rings, a burned valve, or a leaking head gasket, which is less common but more serious.
  6. Mechanical valvetrain fault on cylinder 3, such as a collapsed lifter or worn cam lobe, which disrupts proper air intake and exhaust flow.
  7. Engine control wiring or connector issue at the cylinder 3 coil or injector, including chafed wires or corroded pins.

How to diagnose and fix it

  1. Scan all stored and pending codes with an OBD-II scanner to confirm P0303 is the only misfire code and check for related codes like P0300 or fuel trim codes.
  2. Perform a visual inspection of cylinder 3's ignition components, checking the spark plug, coil, and boot for cracks, carbon tracking, or oil fouling.
  3. Swap the ignition coil from cylinder 3 with a known-good cylinder and see if the misfire follows the coil, which quickly isolates a bad coil without parts cost.
  4. Remove and inspect the spark plug from cylinder 3, checking the gap, electrode wear, and insulator color; replace if worn or fouled.
  5. Test the fuel injector for cylinder 3 by listening for a consistent click with a stethoscope or using a noid light to verify the injector is receiving a pulse.
  6. Run a compression test and, if low, a cylinder leak-down test on cylinder 3 to rule out mechanical faults before replacing ignition or fuel parts.
  7. Check for vacuum leaks at the intake manifold near cylinder 3 using a smoke machine or by spraying carb cleaner around gasket surfaces while watching for idle changes.

What a misfire code does not always mean

  • A P0303 code does not automatically condemn the spark plug or coil; a vacuum leak at the intake runner for cylinder 3 can lean out that cylinder and set the same code without any ignition fault.
  • Technicians often overlook a failing fuel injector because it may still click audibly; a noid light only confirms the electrical pulse, not actual fuel flow, so a flow test or injector swap is sometimes necessary.
  • A flashing check engine light with P0303 means the misfire is severe enough to dump raw fuel into the catalytic converter, and continuing to drive can melt the converter substrate in a short time.
  • Low compression on cylinder 3 from a mechanical fault can mimic an ignition misfire, so a compression test should be performed before replacing multiple ignition parts in a row.
  • Some vehicles set P0303 from a worn spark plug tube seal that allows oil to pool around the plug, causing a misfire that returns shortly after a tune-up if the seal is not replaced.

Can I keep driving with P0303?

If the check engine light is flashing, stop driving as soon as it is safe because a severe misfire can quickly overheat and destroy the catalytic converter. With a steady light and mild symptoms, you can drive cautiously to a repair shop, but avoid heavy acceleration and prolonged driving until the misfire is diagnosed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage