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Can you look in outdoor unit and verify the outdoor temp sensor is indeed connected to control wiring?
The outdoor sensor may have actually failed or a wire could have vibrated loose. Can you give me model on thermostat and outdoor unit if you want to troubleshoot?
OK if you have not done so, make sure this little plug connector has not vibrated off of board in outdoor unit
Take a look when it is light outside.Inside service panel
I will be out of town tomorrow evening but I will open to other experts who may be available after checking this