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Why Is There a Red Light Flashing on My Thermostat?

Chris Helms - Thursday, May 07, 2015

Get answers to your pressing heating and air conditioning system questions.

Thermostat Troubleshooting

When something goes wrong with your heating and air conditioning system, it can be frustrating and downright confusing. And digging out a manual is often easier said than done. What do you do if you have a red light flashing on your thermostat? Here are the answers you might be looking for:

Diagnostic Feature

Many thermostats have a diagnostic feature built in to identify potential issues as they arise in your HVAC system. The flashing red light alerts you to these issues and generally will flash a code indicating what the actual issue is. This allows for an HVAC tech to easily identify what repairs need to be completed.

Causes and Codes

Flashing codes can be indicative of many things. Here are a few of the more common reasons your thermostat might be flashing red:

  • High or low pressure switch is on
  • High discharge temperature
  • Low discharge temperature
  • Defrosting too often or default defrost on
  • Ambient sensor is bad
  • Liquid line sensor is bad

What the System Does

When a red light is flashing on your thermostat indicating a problem, you’ll most likely find the outdoor unit has shut off because of an issue. This is called “lock-out.” When the unit stops running because of an issue it also alerts the thermostat, which triggers the flashing light.

When the unit is locked-out, you can try resetting it by shutting off the thermostat, waiting 30 seconds and then turning it back on. If the system restarts, the reset was successful, and the indicator may have been an error. If the system does not turn back on, service is needed.

If you have any issues with your heating and air conditioning system, including a flashing red light on the thermostat, call Helms Heating and Air Conditioning, any time, day or night at 877-GO-HELMS or 704-821-6255.