Error Code P0305 is described as Misfire Detected in Cylinder #5, which means the vehicle's computer has determined #5 cylinder that is not firing properly.
Error Code P0305 is one of the most common error codes, as engine misfire is a quite a common problem in vehicles. This code signifies misfire problem on cylinder #5. Modern vehicles (vehicles made 1996 on wards) usually are equipped with a computer system called Power train Control Module (PCM), which detects engine troubles, including misfires detection. As soon as it detects the issue, it then automatically triggers the Check Engine light on.
Engine misfire can be caused by a lot of things, such as lack of spark, low compression, poor fuel quality, metering, unmetered air entering the engine, etc., which leads to lack of combustion. When this happens, the computer triggers the Check Engine light and shows a code such as P0305.