AL02 on a MasterTherm heat pump: High Pressure PT - compressor high discharge pressure from pressure transducer
Short answer
On a MasterTherm heat pump, AL02 means: High Pressure PT - compressor high discharge pressure from pressure transducer. 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: High Pressure PT - compressor high discharge pressure from pressure transducer.
Part involved: high pressure transducer / condenser (water side).
When it shows: Control action: Switches off compressor and fan. Operator guidance: Reduce requested heating water temperature, check bleeding/filling of system, check and clean heating water filter; report if repeated.
Likely causes:
- Too high heating water temperature.
- Clogged heating water filter/strainer.
- Insufficient system filling or air in system.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: High pressure PT - compressor high discharge pressure (transducer).
Part involved: condenser / heating water side.
When it shows: Control action: Switches off compressor and brine circulator. Operator guidance: Decrease requested water temperature, check deaeration and filling, clean water strainer; repeated - call service.
Likely causes:
- Too high water temperature.
- Full water strainer.
- Water circulator malfunction.
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.
- Reduce the requested heating water temperature / heating curve on the controller.
- Check the heating system pressure gauge reading is within the installer-specified range.
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.
- Reduce the requested heating water temperature.
- Check the heating system pressure gauge reading.
What the engineer will check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Clean heating water filter/strainer.
- Vent/re-pressurise the heating system.
- Check circulation pump and valves.
- Check HP transducer and setpoint A74.
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 water strainer, vent and refill system.
- Check circulator.
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