Short answer
Check the thermostat, the furnace power switch and breaker, the filter and the furnace door switch, then read the LED error code through the blower door's sight glass. If you smell gas or a CO alarm sounds, leave the house and call the gas utility or 911 first.
Safety first
A gas smell or carbon monoxide alarm is not a furnace-repair call. Get everyone out, then call the utility's emergency line or 911 from outside.
The checklist
Work through these in order.
- Thermostat on Heat with fresh batteries and setpoint above room temperature
- Light-switch style furnace power switch on, breaker not tripped
- Blower door fully closed (a safety switch cuts power if not)
- Filter not clogged
- Outdoor PVC intake/exhaust not blocked by snow, leaves or nests
- Gas valve handle in line with the pipe (open)
Read the error code
Most furnaces built in the last 25 years have an LED on the control board that blinks a code. The chart is usually on the inside of the door. Common codes point to pressure switch, flame sensing or limit faults.
Common causes
The usual failures are a dirty flame sensor, a cracked or worn hot-surface igniter, a stuck pressure switch, a frozen condensate line on high-efficiency units, or a failed inducer motor.
Common questions
Can I clean a flame sensor myself?
Many homeowners do with fine steel wool after shutting off power and gas, but if you're not comfortable working inside the furnace, leave it to a technician.
Why does my furnace click but not light?
Usually a failed igniter, a gas supply problem, or a safety switch that won't prove. Note the LED code before calling.
This guide is general information, not a diagnosis. HVAC systems vary; a licensed technician should inspect yours before repairs.