Found system cycling off on pressure due to fan not coming on - discovered no call for fan from thermostat. Powered R and G together and found fan does operate. Believe system thermostat is no longer supplying power to G on normal call for heat. Cycled system to check operation. Everything runs normally with G forced on. Recommend either replacing thermostat through Nest's warranty or replacement of thermostat. Replaced thermostat per customer request with Honeywell T6 wifi stat and cycled system to check operation. Everything runs normally again!