Thermostat shows E4 and backlight may flicker or unit may reboot intermittently

Safety first: Turn off power to the furnace/air handler at the breaker before testing low-voltage circuits.

Steps to troubleshoot E4 low voltage:

• 1. Measure transformer output: At the furnace control board, measure AC voltage between R and C with a multimeter. It should be around 24VAC (typically 24–28VAC). If significantly lower, the transformer may be failing or overloaded.
• 2. Inspect thermostat wiring: At the thermostat, verify that R (or Rc/Rh) and C wires are firmly connected and not shared with other loads that could cause a drop. Look for any signs of shorting between conductors.
• 3. Check for shorts: Disconnect thermostat wires at the furnace control board and measure resistance between R and C with all devices disconnected. A very low resistance indicates a short in the cable or connected devices.
• 4. Simplify the circuit: Temporarily connect only R, C, and one call wire (e.g., Y for cooling) to the thermostat. If E4 disappears, another connected accessory (humidifier, zoning panel, etc.) may be overloading the transformer.
• 5. Inspect furnace control board: Look for burnt traces or discoloration around the 24VAC transformer and low-voltage terminals on the Goodman board. Replace the transformer or board if visibly damaged.
• 6. Replace thermostat if needed: If 24VAC is stable at the thermostat but E4 persists and the unit reboots, the thermostat's internal power supply may be defective and the thermostat should be replaced.