When your furnace stops working or underperforms, a systematic check of a few simple things can tell you whether you have a quick DIY fix or a problem that needs a technician. Work through the list below in order — most furnace “emergencies” are actually thermostat settings, dirty filters, or tripped breakers. Gas line faults, cracked heat exchangers, and internal electrical failures always require a licensed pro.

Step 1: Check Your Thermostat Settings

Go to your thermostat first. Confirm it is set to HEAT (not COOL or AUTO with a low setpoint), and that the target temperature is higher than the current room temperature. If your thermostat runs on batteries, replace them — a low battery is a surprisingly common cause of intermittent heating failure. If you have a smart thermostat, check that it has not been overridden by a schedule, a geofencing event, or a remote change from another device.

Step 2: Inspect the Air Filter

Pull the filter and hold it up to a light source. If you cannot see through it, replace it immediately. A choked filter blocks airflow so severely that the heat exchanger overheats, causing the furnace’s high-limit switch to shut the system down as a safety measure. Colorado’s dry air, clay-dust soils, and wildfire smoke season mean filters foul faster here than in many other parts of the country — especially during dry, windy spring months. Replace standard 1-inch filters every 1–3 months; thicker 4- or 5-inch media filters can often go longer, per manufacturer guidelines.

Step 3: Check Power and the Furnace Switch

Check your electrical panel for a tripped breaker on the furnace circuit. Reset it one time. If it trips again, do not reset it a second time — a repeatedly tripping breaker signals an electrical fault inside the unit. Also look for the furnace’s own power switch, usually mounted on the wall nearby and resembling a standard light switch. It gets flipped off accidentally more often than you might expect, especially in utility rooms that double as storage.

Step 4: Verify the Gas Supply

Find the manual shutoff valve on the gas line running to your furnace. The handle should be parallel to the pipe (open position). If it is perpendicular, it has been shut off. If other gas appliances — stove, water heater — are working normally, your gas service is live and the issue is likely somewhere in the furnace itself. If no gas appliances are working, contact your gas utility before calling for HVAC service.

Step 5: Read the Error Code

Most furnaces built in the last 20 years have a diagnostic LED light on the control board that flashes a code when a fault occurs. Watch the light and count the flash pattern, then look up the code on the chart inside the furnace door or in your owner’s manual. Common codes indicate a failed ignitor, a pressure switch fault, a limit switch trip, or a flame sensor issue. Write down the code before you call — it helps the technician arrive prepared with the right parts.

  • Thermostat: HEAT mode, setpoint above room temp, fresh batteries
  • Filter: replace if you cannot see light through it
  • Breaker: reset once only; stop if it trips again
  • Furnace switch: confirm it is in the ON position
  • Gas valve: handle parallel to pipe = open
  • Error code: note the LED flash sequence

When to Stop Troubleshooting and Call a Pro

If you smell gas near the furnace, leave the house immediately and call your gas utility from outside. Do not touch any switches. If the furnace is making loud banging, screeching, or grinding noises, shut it off and call for service — running a failing furnace can turn a small repair into a large one. A yellow or orange flame visible through the sight glass (should be blue) suggests incomplete combustion, which is a carbon monoxide risk. Done’s technicians can diagnose and resolve these issues safely, with emergency heating repair available when you cannot wait. For ongoing peace of mind, a furnace maintenance visit each fall keeps the system in shape before Denver’s cold months hit.