E06
Bosch
Output Relay/Triac Failure
Thermostat shows E06 and heating or cooling does not start; internal relay or triac fault indicated.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Failed low‑voltage relay on thermostat PCB, Overcurrent from miswired 24 V circuits, Shorted control wiring to HVAC equipment, Surge or lightning damage to thermostat electronics
How to Fix / Troubleshooting
Safety: This involves control wiring. Turn off power to the HVAC system at the breaker and, if present, the furnace service switch before touching any wires.
- Visual inspection: Remove the thermostat from the wall plate and inspect the back for burn marks or melted plastic. Do not attempt board‑level repair.
- Check for shorts: At the furnace/air handler control board, inspect the thermostat cable for damaged insulation or bare conductors touching metal.
- Verify wiring: Confirm that R, C, W, Y, G, and other terminals are wired according to the Bosch installation manual and your HVAC equipment. Miswiring can overload outputs.
- Test equipment directly: With power off, temporarily jumper R to W (heat) or R to Y (cool) at the furnace control board to confirm the equipment itself operates. Restore power and test. If equipment runs with a jumper but not with the thermostat, the thermostat output is likely defective.
- Replace thermostat: If E06 persists and wiring is correct, replace the Bosch smart thermostat with an identical or compatible model.
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Repair Difficulty
Hard
4/5
Required Part
Bosch Smart Thermostat Main Control Assembly
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