The fixes you can do yourself
1) Set the thermostat fan to AUTO — on ON, the fan runs constantly and blows unheated air between cycles. 2) Replace a loaded filter; a starved furnace overheats, shuts its burners, and the blower keeps running to cool the heat exchanger — cold air is exactly what that protection looks like. 3) Confirm the thermostat is in HEAT mode and set above room temperature; smart thermostats occasionally lose settings after power blips. 4) Check that the furnace switch and breaker are on.
Causes that need a technician
Ignition failure — a dirty flame sensor or failing hot-surface igniter means burners never light or shut down seconds after lighting. Condensate blockage — high-efficiency furnaces lock out when the trap or drain clogs, common in damp Lower Mainland installs. Gas supply or pressure problems. A failed limit switch or control board. And the serious one: a cracked heat exchanger, which can shut down the furnace and is a safety issue — this is why cold-air complaints deserve a real diagnosis rather than repeated resets.
How we diagnose it
We follow the sequence of operation with instruments: verify the call for heat, watch the ignition sequence, measure flame-sensor microamps, check gas pressure and temperature rise against the rating plate. Root cause, one visit — not parts-swapping. That's the engineering-driven approach behind every NEO Service Tech diagnostic across Vancouver, the Lower Mainland and the Fraser Valley.