Thermostat displays E5 and may not control auxiliary heat or emergency heat correctly

Safety: Turn OFF power to the indoor unit and thermostat at the breaker before working on low-voltage wiring.

• Inspect W2/AUX wiring: Remove the thermostat and verify that the W2/AUX wire is properly landed and not frayed or touching adjacent terminals. At the indoor unit control board, confirm the W2/AUX wire is on the correct terminal.
• Check for shorts: Look for bare sections of the W2/AUX conductor touching metal or other wires along its path. Repair with proper wire nuts or replace damaged cable.
• Fuse check: Inspect the low-voltage fuse on the Ruud indoor control board. Replace if blown and re-test. If it blows again when calling for auxiliary heat, there is likely a short in the W2/AUX circuit or a failed heat strip relay coil.
• Test without W2/AUX: Temporarily disconnect the W2/AUX wire at the thermostat and indoor unit. Restore power and see if E5 clears. If it does, the problem is in the wiring or heat strip circuit, not the thermostat.
• Thermostat replacement: If E5 persists even with W2/AUX disconnected and the fuse intact, the thermostat's internal driver may be damaged. Replace the thermostat.