AL06 on a MasterTherm heat pump: Antifreeze - low (heating/cooling) water temperature with risk of freezing
Short answer
On a MasterTherm heat pump, AL06 means: Antifreeze - low (heating/cooling) water temperature with risk of freezing. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 1 source
What it means
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
The manual says: Antifreeze - low (heating/cooling) water temperature with risk of freezing.
Part involved: water temperature probe / backup heater.
When it shows: Control action: Switches off compressor and fan. Operator guidance: Check circuit breaker of built-in electric boiler; check cooling mode not activated by mistake; wait for electric heater to reheat water.
Likely causes:
- Low heating water temperature.
- Long power failure.
- Low water temperature in cooling mode (cooling activated by mistake).
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: Antifreeze water protection - low heating/cooling water temperature.
Part involved: water temperature probe.
When it shows: Control action: Switches off compressor and brine circulator. Operator guidance: Wait for heating of the water by auxiliary heater/bivalent source; increase requested water temperature if in cooling mode.
Likely causes:
- Low heating/cooling water temperature.
- Long electricity supply problem.
- Low water temperature in cooling mode.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Resets automatically after the minimum compressor off time (~6 min) if the same alarm occurs fewer than 3 times per compressor operating hour.
Manual reset: After 3 occurrences in 1 compressor operating hour the unit is blocked (ALARM key lit steadily); manual reset by pressing ALARM key on the alarm display (pGD) or RESET on pGDx Alarm Management.
What you can check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- 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 on the controller that cooling mode has not been selected by mistake.
- Wait for the backup heater to reheat the water; if repeated call the installer.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
- 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 that cooling mode has not been selected by mistake; raise cooling setpoint if cooling.
What the engineer will check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Check the built-in electric boiler circuit breaker (inside switchboard).
- Check water temperature probes.
Who to ask for: heat pump service engineer (refrigeration/controls).
Certification: F-gas certification (refrigerant circuit work); electrical competence for switchboard work.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
- Check backup heater and probes.
Who to ask for: ground source heat pump engineer (refrigeration/controls).
Certification: F-gas certification (refrigerant circuit work); electrical competence for switchboard work.
What you might notice
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 · pages 31, 42-43, 31, 40-41 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026