266 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Low refrig.: Insufficient refrigerant detected at start-up in cooling (1 min after start BT15 must have decreased 15K compared to BT3)
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 266 means: Low refrig.: Insufficient refrigerant detected at start-up in cooling (1 min after start BT15 must have decreased 15K compared to BT3). 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 refrig.: Insufficient refrigerant detected at start-up in cooling (1 min after start BT15 must have decreased 15K compared to BT3).
Part involved: Low refrig.
When it shows: Insufficient refrigerant detected at start-up in cooling (1 min after start BT15 must have decreased 15K compared to BT3).
Likely causes:
- Service valve closed; loose connection on sensor (BT15, BT3); defective sensor BT15/BT3; insufficient refrigerant.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: yes (select 'reset alarm' in the alarm menu).
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). On the RC-HY controller you may select 'reset alarm' once (installation manual: 'in most cases it is enough'); if the red lamp stays or the alarm returns, select 'aid mode' (if enabled) or 'add. heat only' in menu 4.2 and contact the installer.
What the engineer will check
- Per manual 'May be due to': Service valve closed; loose connection on sensor (BT15, BT3); defective sensor BT15/BT3; insufficient refrigerant. 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.81 (printed 79) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026