Block heaters are listed by thread size and by wattage. A heater that is too small will not raise the coolant temperature enough to help a cold start; one that is too large can overheat the coolant locally.
In-line heaters are fitted into a coolant hose and circulate by convection. They are simpler to install than a block heater and are listed with the hose diameter they fit.
Fuel-fired auxiliary heaters are a separate product with their own controllers and dosing pumps. Those parts are listed individually because the controllers fail more often than the heater itself.
Electrical elements should be checked for resistance before replacement. A heater that reads open circuit is dead; one that reads correct resistance but does not heat usually has a supply or earth fault.
