Learn why a gas furnace may not start or complete a heating cycle, including thermostat, power, ignition, flame-sensing, pressure-switch and safety-control issues.
A modern furnace follows a sequence of operation. If a required step or safety condition is not satisfied, the control board may stop the cycle before heat is delivered.
A furnace that appears completely dead has a different troubleshooting path from one that starts an inducer, ignites briefly or lights and then shuts down.
Observing the sequence without bypassing safeties can provide useful information for service.
The furnace may not be receiving a valid request for heat.
Breaker, service switch, transformer, fuse or wiring issues can stop operation.
The furnace must prove proper inducer/venting conditions before ignition on many systems.
Igniter, gas valve, gas supply or control faults can prevent flame.
The burners may ignite and then shut off if flame is not proven.
Overheating or another safety input can interrupt the sequence.
Several different HVAC faults can produce the same homeowner symptom. Proper diagnosis may require electrical readings, refrigerant measurements, airflow testing, static-pressure measurements, combustion testing or control-sequence checks.
Do not bypass safeties, open sealed refrigerant circuits, work on gas piping or perform live electrical troubleshooting unless you are qualified to do so.
Answer a few questions about what your system is doing and get a more targeted list of likely causes, safe checks and related Central HVAC guides.
A furnace may start but fail to complete a normal cycle.
The blower may run without successful heat production.
A missing call can make the furnace appear dead.
Understand the furnace sequence and major components.
When requesting service, include the equipment type, approximate age, when the problem started, whether the system still operates and any unusual noise, water, odor or thermostat behavior you have noticed.
Understand normal system operation before narrowing a symptom.
Identify the components commonly mentioned during diagnosis and repair.