Does this match your problem?
- Furnace blower runs but only pushes cold air
- Furnace clicks but never ignites
- System is completely silent at the thermostat
- Furnace fires briefly then shuts off (short cycling)
Quick check first
Set the thermostat to HEAT and 5°F above room temp. Confirm the furnace power switch (often a light-switch on or beside the unit) is ON. Confirm the gas valve is parallel to the pipe.
Step-by-step troubleshooting
1. Verify the thermostat is calling for heat
Switch to HEAT mode, set the fan to AUTO, and raise the setpoint 5°F above room temperature. Replace batteries in a battery-powered thermostat if the screen is dim or blank.
2. Check the furnace power switch and breaker
Furnaces have a service switch that looks like a wall light switch mounted on or beside the unit. Confirm it's ON. Then check the breaker panel for a tripped 'Furnace' or 'Air Handler' breaker.
3. Inspect the air filter
Pull the filter and hold it to a light. If it's caked, gray, or bowed, you've likely found the cause: restricted airflow lets heat build in the exchanger, which trips the high-limit safety and shuts the burners down, often producing a short-cycling pattern. A loaded filter is behind a large share of no-heat calls. Correct filter sizing and the replacement routine are covered step by step in the DIY Quick-Fix Playbook, or a tech can take care of it on a visit.
4. Confirm the gas is on (gas furnaces only)
Trace the yellow gas line to the shutoff valve near the furnace. The handle should be parallel to the pipe (open). If you've had gas work done recently, other appliances may need to be relit first to purge air.
Warning: If you smell rotten eggs or sulfur, leave the house immediately, call your gas utility from outside, and do not touch switches or breakers.
5. Watch the ignition sequence (gas)
Stand back and watch a start attempt. If the burners light, run 3 to 7 seconds, then die, and the sequence repeats before locking out, that light-and-die pattern is the classic signature of a flame sensor coated in carbon: the furnace can't confirm flame, so it shuts the gas off as designed. Note how many times it retries before lockout, that detail speeds up any diagnosis. The sensor cleaning procedure is covered step by step in the DIY Quick-Fix Playbook, and it's a 20-minute item on a pro visit.
6. Inspect the condensate drain (high-efficiency furnaces)
High-efficiency furnaces (90%+ AFUE) produce condensate, and a clogged drain trips a pressure or float safety that locks the furnace out. Find the small clear tube exiting the unit into a drain or condensate pump and look for standing water, sludge, or a visibly backed-up trap. Water sitting in that line is your answer. Clearing it correctly, without damaging the trap or flooding the burner compartment, is covered in the Playbook, or we can handle it during a visit.
7. Look for a flashing diagnostic code
Modern furnaces flash an LED near the blower door with an error code (e.g. '3 flashes'). Match the pattern to the legend printed inside the door. The code tells you and the tech exactly which safety tripped.
8. Reset the system
Turn the furnace switch OFF for 60 seconds, then back ON. This clears most lockouts. If it locks out again within one heat cycle, it's time for a pro, repeated lockouts indicate a safety is doing its job.
Stop and call a licensed pro if:
- •You smell natural gas at any point
- •The furnace short-cycles (turns on/off every few minutes) after a filter change
- •The blower runs continuously and won't shut off
- •You hear a boom or delayed ignition when the burners light
- •Diagnostic code points to flame sensor, pressure switch, or limit switch failure
- •The furnace is 15+ years old and heat has been weakening