Thermostat display is completely blank and system does not respond

Safety: You will be working near high-voltage components. Turn OFF the main HVAC breakers before removing indoor unit panels. Use caution.

• Check breakers: Verify that the furnace/air handler breaker is ON and not tripped. Reset if necessary.
• Inspect indoor unit fuse: Remove the blower door and locate the low-voltage fuse on the Ruud control board. With power OFF, remove and visually inspect or test with a multimeter. Replace if blown, but investigate for shorts before restoring power.
• Verify R and C at thermostat: Remove the thermostat from the wall plate and confirm that the R and C wires are securely attached and not broken. Tug gently to ensure they are not loose in the wall.
• Measure voltage: With power ON and using a multimeter, measure between R and C at the thermostat base. You should see 24–28 VAC. If not, trace back to the indoor unit to locate the open circuit or failed transformer.
• Transformer replacement: If no 24V is present at the indoor unit control board, the 24V transformer may be failed. Replace it with a Ruud-approved transformer of the same VA rating, or have a technician perform this repair.
• Thermostat failure: If 24V is present at the thermostat base but the display remains blank, the thermostat itself is likely defective and should be replaced.