278 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Sensor fault from heat pump: Input for sensor Tho-A (outdoor sensor) unreasonable 3 times within 40 minutes
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 278 means: Sensor fault from heat pump: Input for sensor Tho-A (outdoor sensor) unreasonable 3 times within 40 minutes. 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: Sensor fault from heat pump: Input for sensor Tho-A (outdoor sensor) unreasonable 3 times within 40 minutes.
Part involved: Sensor fault from heat pump.
When it shows: Input for sensor Tho-A (outdoor sensor) unreasonable 3 times within 40 minutes.
Likely causes:
- Open circuit or short circuit on sensor input; defective sensor.
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': Open circuit or short circuit on sensor input; defective sensor. 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.82 (printed 80) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026