Step 1
Check the Batteries
This is the easiest fix — and often overlooked. Always change thermostat batteries at least once a year. Weak batteries can cause temperature inaccuracy, random shut-offs, or complete system failure.
Step 2
Inspect the Wiring Connections
If you remove your thermostat from its wall plate (sub-base), you’ll see at least two wires connected — usually red and white.
- The red wire (R) carries 24 volts of power.
- The white wire (W) activates the heat when power is sent through it.
When you turn on the heat, the thermostat connects R to W, signaling your furnace to run until the desired temperature is reached.
Step 3
How to Bypass the Thermostat (If You’re Handy)
If your furnace isn’t turning on, you can manually test whether the thermostat is the issue.
With the thermostat off the wall:
- Jump the red wire (R) to the white wire (W) on the sub-base.
This simulates a “call for heat.”
If the furnace starts running, the thermostat is the problem.
You can also do this test directly at the low-voltage terminal strip inside the furnace, where you’ll see terminals labeled R, W, G, Y (and sometimes C).
Step 4
Understanding Common Thermostat Terminals
| Terminal | Function | Description |
| R (Red) | Power | 24V from furnace control board |
| W (White) | Heat | Sends power to furnace when heating is needed |
| G (Green) | Fan | Turns the blower fan on (Auto → On) |
| Y (Yellow) | Cooling | Sends power to outdoor A/C contactor |
| C (Common) | Power return | Used when thermostat is powered by house current instead of batteries |
Pro Tip:
If the C (common) wire is connected incorrectly, it can blow a fuse on your furnace control board or even burn out a transformer if there’s no fuse protection.
Step 5
Advanced Systems (Two-Stage Heating or Cooling)
If your system uses two-stage heating or cooling, you’ll see additional terminals labeled Y2 or W2. These control the second stage of operation for more precise comfort. If your thermostat isn’t configured correctly for your system type, it can cause short cycling or uneven temperatures. When in doubt, have us verify your thermostat’s setup.
When to Call for Help
Ready for Service You Can Count On?
If these checks don’t resolve your issue, it’s best to have a professional take a look. Faulty wiring, power surges, or incorrectly programmed thermostats can damage your furnace or air conditioner.
At Ed’s Heating & Air, we’ll test your thermostat, verify wiring and voltage, and install a reliable replacement if needed — without the upsell.