Could it be thermostat set to fan on?

The blower runs constantly, including between heating cycles. Air feels cold because it's only being moved, not heated. Switch the fan to AUTO.

Could it be dirty air filter?

Restricted return air = furnace overheats and trips a high-limit safety. Burner shuts off; blower keeps running with no heat. Replace the filter.

Older furnaces have a pilot that may have blown out. Newer ones use an electronic ignitor that wears out (~10 years).

Sensor coated in carbon — it can't see the flame, so the gas valve closes. Cleaning takes 10 minutes.

Safety system shuts down combustion to prevent CO leak. Stop using the furnace immediately.

Troubleshooting

Quick answer

Most often a flame sensor problem, a dirty filter, or the thermostat fan switch left on 'On' instead of 'Auto'. Check those three before calling.

Thermostat set to FAN ON
easy

The blower runs constantly, including between heating cycles. Air feels cold because it's only being moved, not heated. Switch the fan to AUTO.
Dirty air filter
easy

Restricted return air = furnace overheats and trips a high-limit safety. Burner shuts off; blower keeps running with no heat. Replace the filter.
Pilot light or ignitor failed
moderate

Older furnaces have a pilot that may have blown out. Newer ones use an electronic ignitor that wears out (~10 years).
Bad flame sensor
moderate

Sensor coated in carbon — it can't see the flame, so the gas valve closes. Cleaning takes 10 minutes.
Cracked heat exchanger
urgent

Safety system shuts down combustion to prevent CO leak. Stop using the furnace immediately.

1 Check the thermostat — switch FAN from ON to AUTO.
2 Pull the air filter and hold it up to a light. If you can't see through it, replace.
3 Look at the furnace's status light through the cabinet window. Note the blink pattern (Google your model + that pattern).
4 If the burner won't fire at all and the inducer fan kicks on for 30 seconds then stops — likely flame sensor or ignitor.

You smell anything burning or sulphurous (gas)
The flame is yellow when it does fire (incomplete combustion = CO risk)
Soot or rust around the flue pipe
Heat exchanger looks discoloured or cracked when you peek inside (safety: don't touch — call us)

Related service

Other symptoms in this category

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