Thermostat displays E5 and backlight may dim or flicker; system may intermittently reboot or shut down.

Safety first: Turn off power to the furnace/air handler before accessing the control compartment.

• Check R and C wiring: Remove the thermostat and verify that the R (or Rc/Rh) and C terminals are firmly connected and not corroded. At the indoor control board, confirm the R and C wires are secure on the Payne control board terminals.
• Measure transformer voltage (advanced): With power on and using a multimeter set to AC volts, measure between R and C at the control board. You should see around 24VAC (typically 24–28VAC). If voltage is low or drops significantly when the system starts, the transformer may be undersized or failing.
• Reduce accessory load: Temporarily disconnect non‑essential 24V accessories (humidifier, UV lights, zoning dampers) to see if E5 clears. If it does, the transformer may be overloaded and should be upgraded per Payne specifications.
• Inspect for loose neutrals or breakers: At the electrical panel, ensure the breaker feeding the furnace/air handler is fully seated and that neutral connections are tight (licensed electrician recommended).
• Thermostat reset: Perform a factory reset from the thermostat menu. If E5 persists with proper 24VAC, the internal power supply on the thermostat may be defective and the thermostat should be replaced.