Furnace or AC suddenly stopped: what to check before you call

What should I check if the furnace suddenly stops?

1. Confirm the thermostat is set to HEAT and the set temperature is above room temperature.

2. Check whether the thermostat display has power.

3. Confirm the furnace service switch has not been turned off accidentally.

4. Check the electrical panel for a tripped HVAC breaker.

5. Look at the air filter and replace it if it is severely restricted and you can do so safely.

If the furnace starts, tries to ignite, then shuts down again, do not keep cycling power over and over. Tell the technician exactly what it does and roughly how long it runs before stopping.

What should I check if the AC stops cooling?

1. Confirm the thermostat is set to COOL and below room temperature.

2. Check whether the indoor blower is running.

3. Look outside to see whether the outdoor unit appears to be operating normally.

4. Check the HVAC breaker and the air filter.

5. Note whether the system runs continuously while the indoor temperature keeps rising.

A system that runs but cannot remove heat from the home may have an airflow, refrigeration or mechanical problem. That is a better reason to schedule a diagnosis than to keep lowering the thermostat.

Which symptoms need prompt attention?

No heat during dangerously cold weather, or no cooling during extreme heat, especially with vulnerable occupants in the home.

Repeated breaker trips or signs of overheating.

A loud delayed-ignition or combustion-related boom.

A gas odor, smoke, burning smell or carbon-monoxide alarm.

Water leaking where it can damage equipment, electrical components or the home.

If you smell gas, see smoke, suspect an electrical hazard, or a carbon-monoxide alarm activates, do not continue troubleshooting the equipment. Move to safety and follow the appropriate emergency or utility guidance.

Should I keep restarting the system?

Usually not. One reset may reveal whether the system had a temporary fault, but repeated resets erase useful diagnostic clues and repeatedly stress a component that is already failing.

A description like “the inducer starts, I hear it try to light, then it shuts off after about 30 seconds” gives a technician far more to work with than “the furnace is broken.”

What information helps us diagnose it faster?

Whether the problem is no heat, no cooling, weak airflow, unusual noise or intermittent operation. What the thermostat is displaying. Whether the indoor blower and outdoor equipment are running. Any error code or flashing light you can see without opening or bypassing safety panels. When the problem started, and whether anything changed right beforehand.

The bottom line

Good pre-service troubleshooting is simple and safe. Confirm the settings and power, observe what the equipment is actually doing, and avoid repeated resets or invasive DIY work. That gives the technician a cleaner starting point and helps you know when a visit is justified.

We answer until 9 PM, seven days a week, and we quote a flat-rate price before any work starts. Furnace repair →  ·  AC repair →

Common questions