Nest thermostat shows E195 and reports that it is shutting down due to power issues, often during peak cooling or heating

Safety: Turn off power to the HVAC system before inspecting wiring or boards.

• Check all low‑voltage connections: Tighten R and C terminal screws at the furnace/air handler and ensure wires are secure at the Nest base.
• Measure under load: With a multimeter, measure voltage between R and C at the furnace board while the system is running. If it drops significantly below 24 VAC, the transformer may be undersized or failing.
• Reduce load: Disconnect non‑essential 24V accessories (e.g., extra thermostats, add‑on modules) to see if stability improves.
• Inspect control board: Look for signs of overheating or discoloration on the control board. Poor solder joints or failing components can cause voltage drops.
• Consider transformer upgrade: Have a technician replace the transformer with a properly sized unit if it cannot maintain stable voltage under load.