E3 error and thermostat indicates low supply voltage or frequently reboots when boiler starts.

Danger: Power input may be mains or SELV depending on model. Always isolate at breaker and verify with a tester before touching wires.

• Step 1 – Check power wiring:
  • Remove thermostat from wall plate.
  • Verify that power input terminals are correctly wired according to Protherm diagram (e.g. 24V AC or 230V AC).
  • Tighten any loose screws.
• Step 2 – Measure supply voltage:
  • With a multimeter, measure voltage at the thermostat terminals under load.
  • Ensure it is within the specified range.
• Step 3 – Inspect cable length and gauge:
  • For low-voltage models, long thin cables can cause voltage drop.
  • Consider upgrading to a thicker cable or relocating the power source.
• Step 4 – Check boiler transformer (if powering thermostat):
  • Ensure the boiler’s internal transformer is functioning and correctly rated.
• Step 5 – Replace thermostat if internal regulator is faulty:
  • If supply is stable but E3 persists, the internal regulator may be damaged.
  • Replace the Protherm smart thermostat.