Possible reasons:
1. The engine may not be getting hot enough. Check if the thermostat has been removed for some reason, if the thermostat is there put it in some boiling water. It should open and then gradually close as the water cools.
If it stays open fit a new one.
2. The small radiator inside the heater (known as the matrix) may be blocked with dirt and restricting the flow of hot water. Try flushing it out with a hose pipe.
3. The temperature control on the dashboard may not be working properly. Look under the dash whilst operating the control and check that it is fully moving a lever on the heater unit.
Dann
September 2007