Problems of complete combustion. 1) CO2 is released into the environment, this is a greenshouse gas and contributes to global warming.
Problems of incomplete combustion. 1) Carbon monoxide is toxic, as it's colourless, tasteless and odorless people cannot detect whether it is around them. 2) Carbon is formed in tiny pieces called particulates, these reach the atmosphere and cause global dimming. It can also provoke people with illnesses such as asthma.