In addition to what Phil said you might also want to check the heater's fan. happend to my dad's Sierra (heater all the way up and freezing cold inside some winters ago)
Shade
November 2007
try getting a new thormostat fitted or it could be your heater valve
phil
November 2007
I can answer this question...
how do i fix my ford ka collection heaters /blowers not working?
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