Heater blowing cold air when idle, warm air once i start driving

Hi,

My check engine light came on "engine malfunction" symbol.  I took it to my
mechanic, he said coolant temp sensor.  He  had to change the housing
because  the sensor isn't sold separately (not sure if this is true).  He fixed it
and everything was fine when i left the garage.  Drove it that night bout 60km
(return).  As i was about to reach home the same light came on.  Took it back
next day and he said it was the coil this time.  Got it fixed and now heater is
blowing cold air when I'm warming up the car, but when i drive it the heat goes
on.  What's going on now?