Thermostat shows E08 and reports low supply voltage; display may dim or reboot intermittently.

Safety: Turn off power to the boiler/furnace/air handler at the breaker before opening access panels. 24 V circuits are low voltage but can still short and damage components.

• Check thermostat wiring: Remove the thermostat and verify R and C wires are firmly clamped and not corroded. Tug gently to ensure they are secure.
• Inspect at equipment: At the Bosch boiler or air handler control board, confirm the thermostat cable is tight on R and C terminals and that no strands are touching adjacent terminals.
• Measure voltage: With power restored and using a multimeter set to AC volts, measure between R and C at the control board and at the thermostat. You should see around 24 VAC. If significantly lower at both points, the transformer may be failing.
• Look for shorts/overloads: Disconnect thermostat wires at the control board and see if the 24 V voltage recovers. If it does, the thermostat or cable may be shorted. If not, another 24 V load (e.g., zone valve, contactor) may be overloading the transformer.
• Call a professional if unsure: Replacing a 24 V transformer or diagnosing control board loads is best done by a qualified technician.