Thermostat shows E10 and indicates low 24V power or brownout; display may dim or reboot

Safety: Turn OFF power to the indoor unit at the breaker before accessing the control compartment.

• Check R and C connections: At the thermostat and at the Ruud indoor control board, verify that the R (24V hot) and C (24V common) wires are secure and not corroded or broken.
• Measure voltage: With power ON and using a multimeter set to AC volts, measure between R and C at the thermostat. A healthy reading is typically 24–28 VAC. If it drops significantly when the system calls for heat or cool, the transformer may be undersized or failing.
• Inspect for shorts: Look for any thermostat wires pinched by sheet metal or contacting sharp edges. Repair insulation or reroute as needed.
• Transformer check: At the indoor unit, measure the secondary of the 24V transformer under load. If voltage is low or fluctuating, replace the transformer with a Ruud-approved part of equal VA rating.
• Thermostat test: If 24V is stable at the thermostat but E10 persists and the thermostat reboots randomly, the internal power supply circuit may be failing. Replace the thermostat.