@Onionman I'm having a similar issue with my unit. It printed fine for several weeks. I suffered the power switch failure as has been plaguing others. Replaced the switch, works fine again.
Prints became less and less dense. Underextrusion began to occur. I noticed my hotend heater was struggling to keep up and it took longer to warm up eventually giving me thermal runaway/time-out errors. The extruder thermistor now reads -15 consistently. New thermistor, same issue. Updated firmware, same issue. Downgraded firmware to prior version, same issue.
Any input would be appreciated from either Creality or from other backers.