P0305 indicates that the powertrain control module has detected incomplete combustion in engine cylinder 5. When a misfire occurs, unburned fuel can enter the exhaust stream and overheat the catalytic converter. The root cause is typically an ignition, fuel, or mechanical problem specific to that cylinder rather than a global engine fault.
The spark plug may be fouled, worn, or damaged, preventing it from igniting the air-fuel mixture consistently in that cylinder.
A failed coil-on-plug, coil pack, or spark plug wire can deliver weak or no spark, causing the cylinder to misfire especially under load.
A clogged, leaking, or electrically failing injector can disrupt the precise fuel delivery required for complete combustion in that cylinder.
Unmetered air entering the intake near cylinder 5 can lean out the mixture enough that the charge fails to ignite reliably.
Worn piston rings, damaged valves, or a leaking head gasket can reduce compression in cylinder 5 below the level needed for consistent combustion.
In very rare cases, the powertrain control module may fail to properly ground the injector or coil circuit for cylinder 5, but this is an uncommon root cause for an isolated misfire.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
It is not recommended. A flashing check engine light indicates a catalyst-damaging misfire is active, and continued driving can destroy the catalytic converter.
You only need to repair the confirmed fault on cylinder 5. Replacing all plugs or coils as preventive maintenance is optional, but it is not required to clear the code.
No. An oxygen sensor monitors exhaust gases after combustion; it cannot cause a misfire in one specific cylinder.
That is almost always wrong. Isolated cylinder misfires are caused by ignition, fuel, or mechanical failures. A PCM replacement should only be considered after every physical cause has been definitively ruled out with proper testing.