If you are having problems with a digital thermostat, there are a few simple tests that can be performed to isolate the problem and help you decide whether to replace the thermostat to fix it.
Step 1 - Check Temperatures
Before getting too involved in testing, check the room temperature on a digital thermometer. If the temperature matches what is being shown on the thermostat, you can verify that the room temp sensor for the thermostat is working. Any discrepancy here indicates a bad thermostat sensor. While the sensor can probably be replaced, it is more cost and time effective to simply replace the entire unit. The next step will be to check the set point calibration.
Step 2 - Calibrate the Room Temp
Digital thermostats can be calibrated, meaning that both the set point and room sensors are brought into balance. Since each thermostat is made differently, you will have to refer to the thermostat instruction manual in order to calibrate the unit correctly. If the manual is not available, call the manufacturer and ask to speak to a service technician; that person should be able to walk you through the simple process. Alternatively, some thermostats have directions for calibration printed inside the cover. Remove the cover and check before calling the manufacturer.
Step 3 - Check Thermostat Connections
If the sensors check out, the next step is to make sure that all of your connections are correct. Look for any corroded wiring, wires that are not making a good connection, and places where a direct short may be impeding thermostat operation. If they are not already being used, put wire nuts on the tips of all wiring connections. Many times, poorly connected wires will vibrate to an inactive state where they appear to be making a connection but no power is able to travel through the circuit. It is important that all connections are tight, and that no stray strands have contact with another wire.
Step 4 - Check the Thermostat
Typically, the wiring inside the thermostat is labeled R, W, Y, G, C. The R lead is usually the 24-volt main power supply for the thermostat, and the C lead is the common ground. Battery operated thermostats will not always have a common ground. Disconnect all of the wires, and then connect the R wire to the W wire. The heating unit should turn on. Next, connect R to Y and G in turn, and verify that the Y turns on the cooling unit, and G activates the blower fan. When you are done with the testing, be sure to hook the wires back up the way they were when you started.
Step 5 – Replace the Thermostat
If you perform all of the simple tests listed here, and the thermostat still does not seem to function properly, the chances are very good that it is the thermostat itself that is faulty. Replacing it with a new thermostat will only take a few minutes of time and may be the only option left if the other tests were all successful. Make sure that the replacement thermostat is the same general size and shape as the old one, and there won't be any problems changing it out.