Thermostat shows E4 and may indicate 'Low voltage' or '24V power error'; display may dim or reboot.

Safety first: Turn off power to the HVAC system at the breaker. Low-voltage circuits can still be hazardous if shorted.

Check the thermostat power:

• Inspect R and C wiring: Remove the thermostat and verify the R (or Rc/Rh) and C wires are firmly connected and not frayed or touching other terminals.
• Measure voltage: With power restored and using a multimeter set to AC volts, measure between R and C at the thermostat sub-base. You should see approximately 24VAC. If it's below ~18VAC or fluctuating, there is a supply issue.
• Check at the indoor unit: Measure 24VAC at the air handler/furnace control board between R and C. If normal at the board but low at the thermostat, suspect a wiring issue in the cable run.
• Inspect for shorts: Look for any thermostat wires pinched by sheet metal or contacting high-voltage terminals.
• Reduce load: Temporarily disconnect non-essential 24V accessories (humidifier, UV light relays, etc.) from the R/C circuit to see if voltage stabilizes.
• If transformer is weak or open: A licensed technician should replace the 24VAC control transformer with a Rheem-approved part and verify proper fuse protection.