DIAGNOSIS Error E195: No power to Rh wire. This means the Nest is not detecting 24V AC from the heating system's R wire. The Rh terminal is for heating power.
FIX Check your furnace or air handler. Ensure the power switch is ON. Check the furnace fuse or circuit breaker. It may be blown. If power is on at the furnace, turn off power to the entire HVAC system at the breaker. Verify the Rh wire is securely connected at both the Nest base and the furnace control board.
IF THAT DIDN'T WORK The 24V transformer in your furnace may have failed. Use a multimeter to test for 24V AC between the Rh wire and the C wire at the furnace control board. If you read 0V, the transformer needs replacement by a technician.
HEADS UP Never connect the Rh and Rc terminals with a jumper wire on a Nest thermostat. The Nest internally manages the connection. Installing a jumper can damage the thermostat.