E10 error and thermostat reports power supply fault or keeps turning off when HVAC starts.

Safety first: Isolate power to the HVAC system and thermostat at the breaker. Low‑voltage circuits can still cause damage if shorted.

• Step 1 – Inspect wiring: Remove the thermostat and check that R, C, and other control wires are firmly connected and not frayed or touching each other. Correct any shorts.
• Step 2 – Measure transformer output: At the HVAC control board or transformer, measure the AC voltage. It should match the rated value (commonly 24 V AC). If significantly low under load, replace the transformer.
• Step 3 – Check wire gauge and length: Very long runs with thin wire can cause voltage drop. If the thermostat is far from the transformer, consider using thicker cable or relocating the transformer.
• Step 4 – Test with spare transformer: Temporarily power the thermostat from a separate known‑good 24 V AC transformer to see if E10 disappears. If it does, the original supply is faulty.
• Step 5 – Replace thermostat: If supply voltage is stable and within spec but E10 persists, the internal power regulator or rectifier on the thermostat PCB may be defective. Replace the thermostat module.