How does a furnace usually fail?
- Ignition. A hot surface igniter that has failed, or a spark ignition problem, and the furnace never lights.
- Flame sensing. The furnace lights, runs for a few seconds, then shuts down. A dirty or failing flame sensor is a common cause.
- Safety switches. Limit switches and pressure switches open for a reason. Restricted airflow, a blocked flue or a condensate problem will all trip them, and the switch is doing its job.
- Blower motor. No air movement means no heat delivery, and it will trip the limit switch as heat builds.
- Control board. Intermittent behaviour that does not fit a simpler explanation sometimes lands here.
Why should you not keep resetting it?
A furnace that locks out and then works after a reset is telling you something. The safety that tripped did so for a reason, and clearing it repeatedly without finding the cause means the underlying fault is still there.
What do you check on the visit?
The ignition sequence, the flame signal, the safeties and why any of them opened, the blower and airflow, the condensate path on a high efficiency unit, and the electrical connections.
Repair or replace
A failed igniter on an otherwise sound furnace is a repair. A cracked heat exchanger is a different conversation, and one where we would explain replacement honestly rather than patching around it. See the repair or replace guide and furnace replacement.
Safety first
If you smell gas, leave the house and call your gas utility from outside before anything else.
Repair or replace: how we weigh it
| What we look at | Points toward repair | Points toward replacement |
|---|---|---|
| What failed | A wear item such as a capacitor, contactor or fan motor | Compressor, leaking evaporator coil or cracked heat exchanger |
| Repair history | First real fault the system has had | Repairs stacking up across recent seasons |
| Condition of the rest | Remaining equipment looks sound | Several components visibly tired |
| Cost comparison | Repair sits well below replacement cost | Repair approaches a meaningful share of replacement |
| Comfort history | System kept the house comfortable until now | It never really kept up, which may be sizing or ductwork |
No formula decides this. We look at your equipment and explain what we find. See the repair or replace guide for the longer version.
Common questions
Why does my furnace start then shut off after a few seconds?
That pattern often points at flame sensing. The furnace lights, does not confirm the flame, and shuts down as a safety response.
Is it safe to keep resetting my furnace?
Not repeatedly. A lockout means a safety tripped. Clearing it without finding why leaves the actual fault in place.
What does furnace repair cost?
A $139 diagnostic visit, waived when you book the repair. The repair itself is quoted before we start.
How do I know if my heat exchanger is cracked?
It needs inspecting. It is not something to diagnose from symptoms alone, and it is a case where we would be straightforward with you about replacement.
Do you work on oil furnaces or boilers?
We work on residential gas furnaces and heat pumps. We do not work on boilers, radiant heat or geothermal.
Reviewed and updated September 2026. Prices shown are owner supplied typical ranges, confirmed after an assessment. If anything here looks out of date, call (615) 400-4993 and tell us.