Your furnace isn't turning on and it's 5°F outside. Before you call a tech—or panic—work through this checklist. Many furnace "failures" turn out to be simple issues you can fix in 10 minutes without a service call.
Step 1: The 5-Minute Quick Checks
These catch the majority of "furnace won't start" calls:
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- Check the thermostat: Is it set to HEAT? Is the temperature set above the current room temp? Did the batteries die? (Yes, this is the #1 call HVAC techs get in October.)
- Check the circuit breaker: Find your electrical panel and look for the furnace breaker — it's often labeled "furnace" or "air handler." If it's tripped (middle position), reset it by flipping fully off then back on.
- Check the furnace power switch: There's usually a light-switch-style power switch on or near the furnace. It gets accidentally turned off more often than you'd think.
- Check the furnace filter: A completely clogged filter can trigger the high-limit safety switch and shut the furnace down. Pull the filter — if it's gray/black and dense, replace it and try again.
- Check the gas supply: Is the gas shut-off valve on the supply line to the furnace in the open (parallel to pipe) position? Did you pay your gas bill?
Furnace Error Codes: Reading the Blinking Light
Modern furnaces have a diagnostic LED that blinks error codes. Look through the small sight window on your furnace cabinet (usually a small clear window near the bottom) for a blinking light pattern. The decode chart is typically printed inside the furnace door or in the manual:
| Common Code | What It Means | DIY Fix? |
|---|---|---|
| 1 flash | System lockout (too many failed ignition attempts) | Reset power, try again |
| 2 flashes | Pressure switch fault | Check condensate drain line (see below) |
| 3 flashes | Pressure switch stuck open | Check inducer motor and vent pipes |
| 4 flashes | High-limit switch open | Replace filter, check for blocked vents |
| 5 flashes | Flame sensed with no call for heat | Call tech — gas valve issue |
| 6 flashes | Low flame signal / dirty flame sensor | Clean flame sensor (DIY, see below) |
| 7 flashes | Gas valve or igniter fault | Call tech |
| Steady on | Normal operation | — |
| No light | No power to control board | Check breaker, power switch, fuse on board |
Note: Exact codes vary by manufacturer and model. Always check your specific furnace's label inside the door.
The Frozen Condensate Drain Line (Minnesota's #1 Winter Culprit)
High-efficiency (96% AFUE) furnaces produce condensate — about 1–2 gallons of water per day in cold weather. This drains through a PVC pipe, often out through the rim joist to the exterior. In a Minnesota winter, that exterior drain line can freeze solid, triggering a pressure switch fault and shutting down the furnace.
Signs: Error code 2 or 3 on the LED; condensate dripping inside the furnace cabinet.
Fix: Carefully thaw the drain line with warm (not boiling) water or a heat gun on low. Long-term fix: reroute the drain to an interior floor drain or install a condensate pump.
Cleaning the Flame Sensor (DIY, 10 Minutes)
The flame sensor is a small metal rod that confirms the burner is lit. Over time, it gets coated with a thin oxide layer and can't detect the flame properly, causing the furnace to shut down after 2–3 seconds of firing. This is one of the most common furnace issues and very easy to fix:
- Turn off power to the furnace
- Find the flame sensor — it's a small rod with a wire attached, positioned in the burner flame path
- Remove the single screw holding it
- Lightly rub the metal rod with fine steel wool or emery cloth until it's shiny
- Reinstall and restore power
When to Call a Tech
Call a licensed HVAC tech if:
- You've worked through the above and the furnace still won't start
- You smell gas at any point
- The CO detector has gone off
- Error code indicates gas valve, igniter, or heat exchanger issues
- The furnace starts but shuts off after 10–30 seconds (flame sensor OR heat exchanger issue)
- It's been running rough for a while and now won't start at all
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