AL01 on a MasterTherm heat pump: Low Pressure - compressor low suction pressure
Short answer
On a MasterTherm heat pump, AL01 means: Low Pressure - compressor low suction pressure. 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: Low Pressure - compressor low suction pressure.
Part involved: low pressure transducer / evaporator.
When it shows: Control action: Switches off compressor and fan. Operator guidance: Wait for the condition to end; if evaporator iced, perform a manual defrost once the unit restarts; contact installer if repeated.
Likely causes:
- Extremely low outdoor air temperature (below -20 C).
- Freezing (icing) of the evaporator.
- Fan operating failure.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: Low pressure - compressor low suction pressure.
Part involved: evaporator / brine circuit.
When it shows: Control action: Switches off compressor and brine circulator. Operator guidance: Switch off unit, clean closed-loop strainer, check brine pressure; repeated problem - call service.
Likely causes:
- Low closed-loop (brine) temperature.
- Evaporator freezing.
- Brine circulator malfunction.
- Full (clogged) closed-loop strainer.
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.
- If heavy icing and the manual permits, start a manual defrost from the pGD display (UP+DOWN together) or Defrost Info screen.
- Check the outdoor unit is not blocked by snow, ice, leaves or debris.
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.
What the engineer will check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Check fan motor and fan circuit.
- Check refrigerant charge/leaks and EEV operation.
- Check low-pressure alarm setpoint (A76) and delay (I18).
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)
- Clean closed-loop strainer.
- Check brine pressure/antifreeze concentration.
- Check brine pump.
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