P0304: Cylinder 4 misfire detected

How to read it

P0304 is one of the standard OBD-II trouble codes stored when the check engine light comes on. SAE J2012, the standard that defines the codes, names it “Cylinder 4 Misfire Detected”; a generic list of OBD-II codes gives the same name.

Also searched as: code P0304, P0304 code.

Steady or flashing check engine light

The federal rule requires the check engine light to blink once per second while misfire is occurring that could damage the catalytic converter. Ford’s F-150 manual says the same of its light: if it flashes, engine misfire may be occurring, and higher exhaust temperatures could damage the catalytic converter.

Why it sets

The federal rule requires the system to detect misfire that raises emissions to 1.5 times the standard, and any misfire that could damage the catalytic converter. Ford’s manual lists temporary causes that can make an engine misfire and light the indicator, among them running out of fuel, and poor fuel quality or water in the fuel.

What the manual says to do

Ford’s F-150 manual says that when the light flashes, the driver should avoid heavy acceleration and deceleration and have the vehicle serviced immediately.

How the light goes out

Under the federal rule, the check engine light may go out once the fault has not come back during three trips in a row under similar engine conditions. The system may erase the stored code if the same code has not been registered again in at least 40 engine warm-up cycles.