IQ Thermostat Error: OD UNIT COMM FAULT.

I have an IQ Drive thermostat with a Frigidaire AC Ecologic unit outside. Each season the IQ thermostat in my house malfunctions. I get error messages from: OD Unit Comm Fault to the message : Needs Configuration. This has happened 3 times in 3 years with the same IQ Drive. Any suggestions?
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"OD UNIT COMM FAULT" most likely means that there is no 24V voltage to feed the inverter control board in the outside unit and it can no longer communicate with the indoor unit. You would need to check for 24V at the terminals of the outside unit's contactor and this is really a task better left for an HVAC professional because it requires opening of the outdoor unit.
The error can also me caused by a fault in the five-wire low voltage control cable that's going from the indoor unit to the outdoor. A bad connection of the ground wire (usually the green wire in the cable) can lead to intermittent and hard-to-explain communication errors.
The inverter circuit board itself can be at fault. They are reportedly sensitive to voltage spikes and may require a "reboot" after a power spike, such as one caused my lighting. You can try to do this yourself by turning the AC off, waiting for the outside unit to shut down and then turning the power to the air handler/furnace off at the breaker panel, waiting 20 seconds and turning it back on. The idea is to cycle the 24V power to restart the control board. Unfortunately, in some cases the transformer feeding 24V is not connected to the same circuit and won't turn off when you shut down the A/C+furnace.
I would try the power cycle trick first and if that didn't clear the issue, call a Nordyne (the original manufacturer of the IQ Drive Split System Air Conditioners under Frigidaire, Maytag, Tappan and other brand names) IQ Drive - certified technician. You can locate them using Frigidaire's service locator page:
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Thank you, Kari. I agree, if repair is imminent, it's best to start with Frigidaire. It's a 3 years old system and there's no reason not to invoke Frigidaire's 10 year warranty. It does not cover the labor cost to troubleshoot and replace the faulty part, but at least the replacement part itself will be provided by Frigidaire (Nordyne) for free.
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