ecobee Wiring Without a C-Wire (Power Extender Kit)
Every current ecobee ships with a Power Extender Kit. It lets you power the thermostat over existing 4-wire cable by relocating G at the furnace. Done correctly, it is the cleanest no-C-wire path for most forced-air systems.
Updated July 2026 · 7 min read · United States
Symptoms
- Only four wires at the thermostat (R, W, Y, G)
- Old battery thermostat with no C terminal used
- Prior smart thermostat had constant power issues
Likely causes
- Four-conductor thermostat cable
- Unused fifth wire never landed on C
What to do
1. Kill power at the furnace switch
Do not work on 24V terminals live. Switch off the furnace and confirm the blower and board LEDs are dark.
2. Install the PEK on the control board
Move the existing G, Y, and R (and W if instructed for your diagram) onto the PEK per ecobee’s heat/cool PEK diagram. Land PEK outputs back on the board terminals.
3. Wire the thermostat for PEK mode
At the wall, the former G conductor becomes C to the ecobee. Follow the on-screen PEK detection prompts — do not force a non-PEK config.
4. Test fan, heat, and cool
From the ecobee equipment tests, run fan, heat, and cool. The G terminal should still energize the blower via the PEK.
Stop and call a pro when
- Two-transformer systems (Rh and Rc both present with no jumper)
- Line-voltage or millivolt systems
- Dual fuel heat pump with fossil backup needing outdoor sensor logic
Related guides
- Nest C-Wire Install — Step-by-step Nest Learning Thermostat C-wire guidance: how to tell if you need a common wire, power connector options, and Rh/Rc/W/Y/G labeling.
- Honeywell T9 Wiring — Honeywell Home T9 terminal mapping for conventional furnaces and heat pumps, including O/B, Aux/E, and C-wire requirements.
- Trane Code 67 — What Trane comfort control code 67 means for heat pumps and AC, when it is normal behavior, and when a control board or sensor is actually failing.
FAQ
Does the PEK disable the fan-only mode?
No. When installed correctly, fan-only still works because the PEK recreates G control.
Can I skip the PEK if I have five wires?
Yes. Land the spare on C at both ends and skip PEK — that is always preferable when a conductor is free.
Need a licensed tech instead?
If the guide says to stop — or the part swap did not clear the fault — get free quotes from licensed local HVAC contractors.
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