I have a new motor, thermostat is only a year and a half old, heat comes on, burners won't stay lit. Getting faint heat from vents on first floor level of house

Exact model number of furnace?
Also post code (number of flashes) on circuit board.

Typically if the burners ignite but fail to remain lit then I start with the flame sensor. Pull it out, clean it with some sandpaper and reinstall. If this is the problem then cleaning it will have you all warm and toasty in no time.