I have a Day & Night Gas Furnace that a month ago had a pc board replaced and later on the thermostat was also replaced, because it was only throwing cold air. Now, after those fixes it's still not working well. It starts throwing hot air but later on it starts throwing room temperature air for awhile and then it starts the cycle again; it never reaches the desired temperature.

The technician said that this is happening because the coil is too dirty that it doesn't let the air circulate properly. I don't believe him because this never happened before the board malfunctioned. Could that be possible?

I'll appreciate your help.