Ecobee
Insufficient Conductors / Missing Common Wire
Thermostat does not power on after installation with only two wires available
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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
Only R and W wires present from old mechanical thermostat, PEK not installed where required, Incompatible system (millivolt, line-voltage) with ecobee
How to Fix / Troubleshooting
Safety: Turn off power to the furnace/air handler before modifying wiring. Do not connect ecobee directly to line-voltage or millivolt systems.
- Step 1 – Identify system type: Confirm your system is a standard 24V HVAC system, not line-voltage baseboard or millivolt.
- Step 2 – Check for spare conductors: At the furnace, inspect the thermostat cable. Often additional unused wires are available; one can be connected to C and used at the thermostat.
- Step 3 – Install PEK if compatible: If no spare wire is available and your system is compatible, install the ecobee Power Extender Kit at the furnace following the wiring diagram.
- Step 4 – Run new cable if needed: If PEK is not compatible (e.g., multi-stage, communicating systems), run a new multi-conductor thermostat cable that includes a dedicated C wire.
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Repair Difficulty
Hard
4/5
Required Part
Ecobee Power Extender Kit (PEK) or Thermostat Cable
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