Furnace won't ignite
When a furnace gets a heat call but never lights or proves flame, check safe outside items first and know which signs mean stop and call.
Furnace symptoms
Choose the problem you see below. Each guide starts with checks you can make from outside the furnace and explains when to call for service. If a code is showing, use the brand and model reference to read it.
A symptom narrows the question but does not diagnose a part. Code meanings depend on the exact brand, model and control, and these guides do not replace a technician's inspection.
Furnace not heating
If rooms stay below the setpoint, sort out whether the furnace is not responding, running weak, or missing certain rooms before calling.
Furnace short cycling
Short cycling means heat runs that stop early and restart before the house reaches the setpoint. Learn what to record and when to call.
Furnace blowing cold air
Cold air from the vents can come from the fan setting, normal start and end delays, or a burner shutdown. Sort out which one you have.
Furnace pressure switch error
A pressure switch code means draft feedback did not match what the control expected. It does not prove the switch failed. Here is what to record.
Furnace limit switch tripped
A limit code can mean overheating from low airflow, or on some controls a rollout or blocked-vent switch on the same circuit. Know the difference.