E06 error and thermostat reboots or freezes frequently when calling for heat or cool

Warning: 24VAC circuits can still cause shock. Turn off power to the furnace/air handler at the breaker before handling wiring.

• Step 1 – Check C‑wire presence:
  • Remove the thermostat and confirm that the C terminal has a firmly connected wire.
  • If no C‑wire is present and the thermostat is battery‑free, the device may be under‑powered.
• Step 2 – Measure transformer voltage:
  • At the furnace control board, measure AC voltage between R and C with a multimeter.
  • It should be around 24VAC (typically 24–28VAC).
  • If significantly lower, the HVAC transformer may be failing or overloaded.
• Step 3 – Inspect thermostat cable:
  • Check for damaged or pinched thermostat cable along its run.
  • Loose connections at wire nuts or furnace board terminals can cause voltage drop; tighten as needed.
• Step 4 – Reduce load on thermostat power:
  • Disconnect any add‑on modules powered from the thermostat (e.g., external sensors that draw power from R/C).
  • Ensure the thermostat is not powering other devices from its 24V supply.
• Step 5 – Replace thermostat if internal regulator is faulty:
  • If supply voltage is stable and wiring is correct but E06 persists, the internal DC power regulator on the main PCB may be defective.
  • Replace the thermostat or main control PCB.