Previously working Nest T-stat E298 error

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Previously working Nest T-stat E298 error

Hi,    My t-stat (not sure what gen it is) has worked for a couple years and today it's showing a E298 error. I've checked the wires on t-stat and hvac unit, all appears ok. Breakers aren't tripped, it worked fine this morning. I've reset the t-stat and tried reconnecting to Google home with no luck. I will check the drain from outside later, the drain inside appears clear though. Any suggestions in not thinking about?