157 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Low lp cool: The low pressure transmitter in the outdoor unit is showing a value too low (R410A manual: protection against freezing in water HX during cooling)
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 157 means: Low lp cool: The low pressure transmitter in the outdoor unit is showing a value too low (R410A manual: protection against freezing in water HX during cooling). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Low lp cool: The low pressure transmitter in the outdoor unit is showing a value too low (R410A manual: protection against freezing in water HX during cooling).
Part involved: Low lp cool.
When it shows: The low pressure transmitter in the outdoor unit is showing a value too low (R410A manual: protection against freezing in water HX during cooling).
Likely causes:
- Poor flow; significant wind effect.
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: yes.
Manual reset: no.
What you can check
- Note the alarm number/text shown on the display. Check the consumer-unit breakers/isolators for the indoor and outdoor units are on. Check the outdoor unit is not blocked by leaves, snow or ice and its drain is clear (look only - do not open the casing). Check radiator/underfloor thermostats in the house are not all closed (user manual). Check the heating system pressure gauge reads in the normal range (about 0.5-1.5 bar when cold for Hydrolution per user manual). This is an information message; it resets automatically when the cause is removed.
What the engineer will check
- Per manual 'May be due to': Poor flow; significant wind effect. Refrigerant/outdoor-unit work (service valves, charge, sensors Tho-R/A/D/S, LPT, 63H1, fan motor, inverter/control PCBs) must be done by an F-gas certified engineer.
Who to ask for: Heat pump / refrigeration engineer (manufacturer-trained).
Certification: F-gas certification for refrigerant-circuit work; electrical competence (Part P/18th Edition) for PCB/wiring.
What you might notice
Sources
- Installation Manual - Air to Water Heat Pump using R32 refrigerant, Hydrolution (HM) ↗ · PSC012D169 (202201) · page PDF p.86 (printed 84) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026