Skip to main content
Universal Air Systems

Troubleshooting

Furnace Won't Turn On

Quick answer

Check the thermostat batteries and the furnace power switch first — they fix half of these calls. Then check the breaker, then the door safety switch.

Common causes

  • Dead thermostat batteries

    easy

    Common, easy fix. Replace the batteries or the thermostat won't signal the furnace.

  • Furnace switch flipped off

    easy

    There's a regular light switch near the furnace that controls power to it. Sometimes flipped accidentally.

  • Tripped breaker

    easy

    Furnace blower has its own breaker. A tripped breaker = no power.

  • Door safety switch stuck

    moderate

    The cabinet door has a button safety. If pushed-in but not making contact (door panel slightly off), the furnace won't run.

  • Failed transformer

    moderate

    The 24V control transformer can fry. No control voltage = no thermostat signal. Tech-only repair.

DIY first

  • 1 Walk to the thermostat. Replace the batteries if it's not lit.
  • 2 Confirm the wall switch on or near the furnace is ON.
  • 3 Check the electrical panel for a tripped breaker labeled FURNACE or HVAC.
  • 4 Re-seat the furnace door panel firmly — sometimes a slightly-loose door doesn't engage the safety.

Call us if:

  • All of the above fail
  • The breaker re-trips immediately when you reset it (electrical short — don't keep resetting)
  • You smell gas anywhere near the furnace

Related service

Furnace Repair

Other symptoms in this category

Open 24/7

Diagnosis pointing somewhere serious? Call now.

Most GTA calls get same-day service. Call, text, or request a fixed quote — we'll come back with a real ETA.

Call (416) 258-2460 Text (416) 258-2460

Mississauga, ON · Greater Toronto Area and up to 2 hours out — London, Kitchener, Barrie, Kingston, Niagara.

Call (416) 258-2460 Text

(416) 258-2460 · 24/7