It's freezing outside and your furnace won't turn on. Before you panic, there are several things you can check yourself — some take less than 30 seconds and cost nothing. Here are the 8 most common reasons a furnace won't start, from easiest to most complex.
1. Thermostat Is Set Wrong (Free Fix)
This is the #1 reason we get emergency calls that turn out to be simple fixes. Check that your thermostat is set to "Heat" mode (not "Cool" or "Off"), the temperature is set higher than the current room temperature, and the fan is set to "Auto" rather than "On." If you have a smart thermostat, check that a schedule or eco mode hasn't overridden your settings.
2. Thermostat Batteries Are Dead (Free Fix)
If your thermostat screen is blank or flickering, replace the batteries. Most use AA or AAA batteries. A dead thermostat can't send the signal to turn on your furnace.
3. Dirty Air Filter (Under $10 Fix)
A severely clogged filter can restrict airflow so badly that the furnace overheats and shuts down as a safety measure. Pull out your filter — if you can't see light through it, it's time to replace it. This is the most common preventable furnace problem in Denver, where dry dusty air clogs filters faster than in humid climates.
4. Circuit Breaker Tripped (Free Fix)
Check your electrical panel for a tripped breaker. The furnace breaker should be labeled. Flip it fully off, then back on. If it trips again immediately, there's an electrical issue — call a technician.
5. Furnace Power Switch Is Off
Most furnaces have a dedicated power switch that looks like a light switch, usually on the side of the furnace or on a nearby wall. It can get accidentally bumped off. Make sure it's in the "On" position.
6. Pilot Light Is Out (Older Furnaces)
If your furnace is over 15 years old, it may have a standing pilot light. If the pilot is out, follow the relighting instructions on the furnace's label. If it won't stay lit, the thermocouple likely needs replacement ($150–$250 repair).
7. Igniter Failure (Newer Furnaces)
Modern furnaces use electronic igniters (hot surface ignition) instead of pilot lights. These wear out after 3–7 years. Signs: you hear the furnace click on and the blower run, but no heat comes out. This is a common repair — $150 to $300 for parts and labor.
8. Gas Valve or Control Board Issue
If none of the above fixes work, the problem is likely a failed gas valve ($350–$600), control board ($300–$700), or draft inducer motor ($300–$600). These require a licensed HVAC technician to diagnose and repair safely.