E8
Ecobee Power Extender Kit Wiring / Detection Error

Thermostat displays E8 and indicates a problem with the power extender kit (PEK)

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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

Incorrect PEK wiring at furnace control board, Loose PEK connector at thermostat, Using PEK with incompatible multi-stage or communicating system, Shorted thermostat cable pair used by PEK

How to Fix / Troubleshooting

Safety: Turn off power to the furnace/air handler at the breaker before touching any wiring or the PEK.

  • Step 1 – Verify PEK installation: Open the furnace/air handler panel and locate the ecobee PEK. Compare each wire connection to the ecobee PEK installation diagram. Ensure R, G, Y, W, and C are correctly landed.
  • Step 2 – Check thermostat side: Remove the ecobee from the backplate and confirm the labeled wires (R, G, Y, W, C) match the PEK instructions. Reseat the thermostat on the backplate firmly.
  • Step 3 – Inspect cable condition: Look for damaged thermostat cable, crushed insulation, or signs of shorting between conductors. Repair or replace damaged cable.
  • Step 4 – Consider adding a dedicated C wire: If possible, run a new cable from the furnace to the thermostat to provide a true C wire and remove the PEK entirely. This is more reliable long-term.
  • Step 5 – Replace PEK if faulty: If wiring is correct and E8 persists, the PEK module may be defective. Replace it with a new ecobee PEK or convert to a C wire.
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Repair Difficulty

Hard 4/5

Required Part

Ecobee Power Extender Kit (PEK)
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