AL03 on a MasterTherm heat pump: High Discharge Temperature - high compressor discharge gas temperature
Short answer
On a MasterTherm heat pump, AL03 means: High Discharge Temperature - high compressor discharge gas temperature. 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 Discharge Temperature - high compressor discharge gas temperature.
Part involved: compressor discharge temperature probe.
When it shows: Control action: Switches off compressor and fan. Operator guidance: Reduce requested heating water temperature, check bleeding/filling, clean heating water filter; report if repeated.
Likely causes:
- Insufficient refrigerant charge or minor leak.
- High heating water temperature.
- Extremely low outdoor air temperature.
- Sensor failure (see AL10).
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: High Discharge Temperature.
Part involved: compressor discharge probe.
When it shows: Control action: Switches off compressor and brine circulator. Operator guidance: Decrease water temperature, check deaeration/filling, clean strainer; repeated - call service.
Likely causes:
- Too high water temperature.
- Full water strainer.
- Water circulator malfunction.
- Slight refrigerant leak.
- Temperature probe problem (AL10).
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 on the controller.
- Check the heating system pressure gauge reading.
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)
- Leak test and check refrigerant charge.
- Check discharge probe (AL10).
- Clean heating water filter, vent system.
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)
- Leak test/charge check.
- Probe check.
- Clean strainer.
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