Can a Bad Thermostat Cause the AC Unit to Shutoff but Let the Air Handler Keep Running?

A faulty thermostat can indeed lead to the air conditioning (AC) unit shutting off while the air handler continues to operate. Understanding the potential issues and how to diagnose them can save you from unnecessary repairs and ensure your comfort during summer months.

Diagnosing a Bad Thermostat

To test if your thermostat is the problem, you can perform a simple check. First, remove the thermostat from the wall and jump the red wire to the yellow R to Y terminal at the wall plate. If the outdoor AC unit turns on, it indicates that your thermostat is faulty.

Additionally, check your circuit breaker to ensure you have primary power running to the system. This step is crucial as power issues can also cause similar symptoms.

Understanding Common Issues

This is a relatively common problem. In my 20 years as a natural gas technician, I encountered this issue 1 or 2 times a week. On the few occasions where there was an actual problem, it resulted from various switch failures or thermal sensor malfunctions. Here are some examples:

The circuit board failed, short-circuiting, causing the fan to not shut off. A fan bearing failure led to a fire incident in the closet.

Other issues are less severe, often resulting from incorrect usage. Many people set their thermostat to ON for continuous fan operation, misunderstanding the function of the switch. The fan can stay on indefinitely without the thermostat being in demand. To resolve this, simply switch the fan control to AUTO.

Thermostat Switches and Combinatorial Failures

Thermostats have distinct switches for controlling the air handler and the AC compressor. Various combinations of switch and thermal sensor failures can cause the air handler to operate or shut off, and the same with the compressor.

It is possible for the air handler switch to be stuck on, but it's also crucial to remember that most thermostats have a fan control option allowing the fan to operate whenever the AC compressor is active or continuously. The switch positions are usually marked AUTO and ON.

Further Diagnostics

If you determine that the yellow wire is not powered, another possibility to explore is theStatus of the green wire. If the green wire is hot, it could indicate that the air handler fan is operating, even without power to the outdoor unit. To confirm this, disconnect the yellow wire from the Y terminal on the thermostat and connect it to the green terminal. This will help you determine if the Y terminal is the cause of the issue.

Understanding and addressing thermostat issues can significantly improve the performance and longevity of your air conditioning system. Regular maintenance and proper usage can prevent many common problems.