The heater has always been a bit cool. We replaced the double thermostat but it made no difference. Next plan is to replace the heater matrix in case it is blocked. Just been reading up about the cooling system on these engines - pretty complicated aren't they. I'm wondering what part the electric coolant pump plays. Does it circulate water through the heater before the mechanical pump starts working? If the electric pump was faulty could this be the cause of a tepid heater? If it was faulty would it show an error code?