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How to Test or Bypass a Thermostat

If your heating equipment isn’t working properly, maybe the thermostat is defective.

Here’s how to see if your thermostat works:

1 With the power to the furnace off, remove the thermostat or thermostat cover to expose the wires. Note that the wires should be screwed to terminals marked R (red), W (white), G (green), Y (yellow), and C (common). Some thermostats have only Red and White or Red and Green connected. Make a note of which wires are connected to the terminals or, better, take a photo with your smart phone.

repairing thermostat©Don Vandervort, HomeTips

Many electronic thermostats are made in two pieces: baseplate and body. With this type, you pull the body straight out to remove it.

2 Unscrew and remove the wires from their terminals. Do not let them fall back in through the hole in the wall (wrap them around a pencil if necessary). If there are more than two wires, pick Red and White—normally these are standard colors for power and heat.

thermostat wires

Electronic thermostat’s wires are attached to terminal screws on the base.

3 Twist the bare ends of the two wires together. Make sure none of the other wires are touching these wires or each other.

4 Turn the power to the furnace back on. If the blower goes on and the furnace burner ignites, the thermostat was improperly connected to the wires or is defective. Replace it as discussed in the article How to Install an Electronic Thermostat.

5 If the burner does not ignite, check the continuity of the wires from the thermostat to the furnace. (If you’re checking the air conditioning, only the blower will go on.)

6 Check and tighten the terminal screws at all wire connections.

If the AC doesn’t work, go through this same process with the Red and Yellow. If the fan isn’t working, repeat with the Red and Green.

It’s possible to use a voltmeter to check a thermostat without disconnecting wires, as shown in the following video.

About Don Vandervort
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Don Vandervort has developed his expertise for more than 30 years, as Building Editor for Sunset Books, Senior Editor at Home Magazine, author of more than 30 home improvement books, and writer of countless magazine articles. He appeared for 3 seasons on HGTV’s “The Fix,” and served as MSN’s home expert for several years. Don founded HomeTips in 1996.

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