Your gas furnace relies on two quiet guardians to keep your home safe and your system running. One is the thermocouple. It sits right next to the pilot burner. Its job is simple. If the flame dies, this device cuts the gas supply immediately. No flame, no gas. That is the deal.
The other is the limit switch. It lives just below the plenum, monitoring heat levels. If the furnace gets too hot, it kills the burner. It also manages the blower, turning it off when things cool down enough. When these parts fail, you get cold spots, weird noises, or a pilot that keeps dropping.
Here is how to handle the most common issues without calling a pro.
Pilot Light Won’t Stay Lit
A pilot light that won’t stay lit usually points to a bad thermocouple. Maybe it is just loose. Maybe it is dead. Start with the easy fix. Grab a wrench.
Tighten the thermocouple nut. Do not go crazy here. Just a slight turn. Too much pressure can strip threads or crack the tube. After you tighten it, try to light the pilot again. If it stays lit, you are lucky and done. If it dies again, the thermocouple is shot. You need to replace it with an exact match.
Here is the process for swapping it out.
- Disconnect the lines. Unscrew the copper lead and the connection nut inside the threaded joint on the gas line. Then, move to the mounting bracket. Unscrew the nut holding the steel tube in place.
- Install the new part. Slide the new thermocouple into the bracket hole. Orientation matters. The steel tube goes up into the flame. The copper lead goes down to the gas valve. Screw the bracket nut back over the tube.
- Reconnect. Push the connection nut to the gas line thread. Ensure everything is clean and dry before you start turning. Screw the nut in tight, but not overly tight. Hand-tight plus a little wrench torque is the sweet spot for both the bracket and the gas connection.
If the pilot still won’t stay lit after a proper replacement, you might have a dirty pilot or an issue with the gas valve itself. But 90% of the time, it is the thermocouple.
Blower Running Non-Stop
Now for the noise. If your blower runs continuously, it can get annoying. It also wastes energy. First, check the thermostat. Is the fan switch set to “ON” instead of “AUTO”? Flip it to AUTO. Test it. If the blower stops when the heat cycle ends, you were done.
If the switch is already on AUTO and the fan keeps running, the limit switch needs adjustment. This switch controls when the fan kicks in and when it shuts off. If the calibration is off, the fan might not get the signal to stop.
To fix this, you need to look at the dial inside the control.
Adjusting the Limit Switch
Remove the cover of the limit switch control. Underneath, you will see a toothed dial. It has two sides. One side is marked LIMIT. Do not touch this side. Adjusting the limit side changes the temperature at which the furnace shuts down, which is a different repair entirely.
The other side is marked FAN. This is where you work. You will see two pointers on the fan side. One sets the temperature for the blower to turn on. The other sets the temperature for it to turn off.
These pointers should sit about 25 degrees apart. A standard setup looks like this:
- Upper Pointer (On): Set to approximately 115 degrees Fahrenheit (46.1 degrees Celsius).
- Lower Pointer (Off): Set to approximately 90 degrees Fahrenheit (32.2 degrees Celsius).
Adjust the pointers to these settings. This ensures the blower kicks in only when there is heat to distribute and shuts off shortly after the burner stops. It prevents that
