Start by confirming there is a call for heat. Set the thermostat above room temperature and allow the equipment its normal startup delay.
Check mode and power
The thermostat should be in Heat mode. Check the equipment service switch, electrical panel, and thermostat batteries where applicable. Reset a tripped breaker once; if it trips again, stop.
Look at the filter and service doors
A severely loaded filter can contribute to protective shutdowns. Some furnaces will not run if an access panel was not fully replaced after maintenance.
Observe without disassembling
Error codes, status lights, clicks, and repeated ignition attempts are useful evidence. Do not open combustion compartments or bypass sensors.
Gas odor changes the priority
If you smell gas, stop trying to start the system, avoid switches and ignition sources, and follow your gas utility’s emergency procedure. A carbon-monoxide alarm also requires immediate action according to the alarm and emergency guidance.
Know when the next step is service
If the thermostat is calling, power is available, and the equipment still never starts, ignition, motor, control, pressure, or fuel issues may be involved. That is qualified diagnostic work.
This path is specifically for “does not start.” A heater that runs but delivers little or no heat is a separate troubleshooting problem.
Sources
- U.S. Department of Energy — Home Heating Systems — Heating-system operation, efficiency, controls, and maintenance.
- U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center — Carbon-monoxide hazards, alarms, fuel-burning appliances, and emergency response.