P0305 on a Mazda: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

If the check engine light is flashing, stop driving — a severe misfire can damage the catalytic converter. A steady light is usually safe for a short drive to a shop.

What P0305 means on a Mazda

On a Mazda, P0305 means the engine computer detected a misfire on cylinder 5. Mazda's V6 engines, like the 3.7L in the CX-9, have six cylinders, and cylinder 5 is on the bank opposite cylinder 1. Because the code names one cylinder, the cause is usually its ignition coil, spark plug, or injector.

Is P0305 common on Mazdas?

P0305 appears on Mazda V6 models like the CX-9 and older V6 Mazda6. Worn spark plugs are the most common cause at higher mileage, and failing ignition coils come next. Clogged or leaking injectors are less common but can misfire one cylinder.

Commonly reported on: CX-9, Mazda6.

Symptoms

Check engine light on (flashing if severe)
Rough idle or shaking
Loss of power or hesitation
Poor fuel economy

Common causes

most common: Worn or fouled spark plug on cylinder 5
common: Failing ignition coil on cylinder 5
less common: Clogged or leaking fuel injector
less common: Wiring or connector issue at the coil
less common: Low compression from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace the spark plug on cylinder 5 (or all plugs)	Yes	$40–150
Replace the ignition coil on cylinder 5	Yes	$80–250
Replace or clean the cylinder 5 injector	Mechanic	$200–500
Run a compression test if the misfire returns	Mechanic	$100–200