Shortly after the combustion fan starts the furnace should try to ignite. If the unit does not try to light then it could mean the proving switch for the fan is not connecting or intermittent.
Condensing Gas Furnaces have drains connected near the bottom of the furnace. If this drain line plugs up , it will prevent the furnace from operating. Make sure the drain is clear.
The combustion exhaust from a condensing furnace is through a PVC pipe outside the house. If this get blocked by snow , ice , debris ect… it will prevent the unit from firing. The outlet is often close to the ground, 18″ or more. Birds have been known to stuff twigs in the outlet.
Have a listen to the combustion fan when it is trying to light. If it is smooth and fairly quiet then it should be ok. If it sounds rough or laboured at all, it could mean worn bearing which can cause problems.
Place a hand on the motor after it is shut off. Does it feel hot? You should be able to comfortably hold your hand on the motor with out burning. If you cant hold your hand there, its too hot and has likely failed.
Great care is needed to replace a combustion fan. Improper installation can result in fuel gas leaking into the house and have serious consequences.