P0305 code definition
Cylinder 5 Misfire Detected
What the P0305 code means
P0305 is a generic OBD-II code that the engine control module (ECM) detected a misfire on cylinder number 5 of the engine.
What causes the P0305 code?
- The spark plug is not firing for cylinder number 5 due to fouling or cracks in insulation.
- The ignition coil for cylinder number 5 has stopped firing.
- The spark plug wire or plug boot is leaking ignition spark to ground for cylinder number 5.
- The valve cover is leaking oil into the spark plug holes and shorting the spark plug firing for cylinder number 5.
What are the symptoms of the P0305 code?
- The Check Engine Light will be illuminated and the code is set in ECM memory.
- The Check Engine Light may flash during a misfire event indicating catalyst damage may occur.
- The engine will have a misfire causing a buck or jerk and a rough idle.
- The engine may lack some power during acceleration while the misfire is active.
How does a mechanic diagnose the P0305 code?
Scans codes and documents the freeze frame data in order to verify the problem
Clears the engine and ETC codes to verify concern comes back
Road tests to verify the misfire to cylinder 5
Removes cylinder 5 ignition coil or plug wire and checks for evidence of spark tracking on outside of coil or plug boot. Also looks for oil contamination from a leaking valve cover
Removes and checks the cylinder spark plug and replaces or switches to a different cylinder to see if misfire moves to the other cylinder
Checks cylinder compression if no ignition problems are found
Common mistakes when diagnosing the P0305 code
Not switching the coil to a different cylinder and rescanning to see if the misfire moves to another cylinder to check if it is the coil that is bad.
Not switching one part at a time with a different cylinder to isolate the failed part.
How serious is the P0305 code?
A misfiring cylinder can cause the engine to run excessively rich and can damage the catalyst. The misfire will cause the engine to run rough and hesitate on acceleration resulting in decreased performance and fuel economy.
What repairs can fix the P0305 code?
Replacing the faulty spark plug or replacing all spark plugs as a set if failure was due to a worn out plug.
Replacing the leaking valve cover gasket then replacing the failed coil, plug wires, and plugs.
Additional comments for consideration regarding the P0305 code
Code P0305 is triggered when the crankshaft sensor does not get an acceleration of the crankshaft from cylinder number 5 power stroke indicating the cylinder is not contributing to engine power.
Need help with a P0305 code?
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This article originally appeared on YourMechanic.com as P0305 OBD-II Trouble Code: Cylinder 5 Misfire Detected and was authored by Jay Safford.