E! on a MasterTherm pCO5/pGD controller alarms - air/water units: E! on main screen - electric (auxiliary) heater safety thermostat activated; heater cannot switch on

What it means

The manual says: E! on main screen - electric (auxiliary) heater safety thermostat activated; heater cannot switch on.

Part involved: auxiliary heater safety thermostat (trips >70 C).

When it shows: Heater safety thermostat opened.

Likely causes:

• Heater overheating.
• Clogged water inlet strainer.
• Insufficient heating water filling/pressure.

Is it safe?

The heat pump keeps running with some functions stopped.

Resetting itself: No.

Manual reset: Manual reset of thermostat button under plastic cover inside switchboard (power isolated).

What you can check

• Read and note the alarm (ALx code/text) and the alarm counter shown on the pGD/pGDx display.
• Check the heat pump breaker/isolator in the consumer unit is on.
• Check the heating system pressure gauge reading.
• Inform the installer (the reset is inside the unit casing).

What the engineer will check

• Isolate power, open front cover, reset heater safety thermostat.
• Check water strainer, filling and pressure.

Who to ask for: heat pump engineer / electrician.

Certification: Electrical competence for switchboard work.

What you might notice

• ALARM key backlight flashes (auto-reset pending) or lit steadily (unit blocked)
• Alarm shown on pGD/pGDx display

Sources

• MasterTherm pCO5/pGD digital controller - OEM instruction manual, heat pumps AirMaster, AquaMaster, EasyMaster, BoxAir (Documentation PGD1-MANUAL, Edition 41, sw_11) ↗ · Edition 41 / sw_11 · page 36, 44 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026