Heat Pump Error Codes: What They Mean and Your First Responses (2026)
Updated: 2026-10-01 · ~3 min read
The big picture: what error codes protect
Modern heat pump controllers log a lot of "errors" that are really protection events: the compressor stops because something (flow, pressure, temperature, electrical load) went out of range. Three facts prevent most panic:
- Most codes self-clear. A single blip after a defrost, a power dip, or a sensor wobble is normal — check whether it recurs.
- Codes cluster by subsystem: high-pressure / low-pressure (refrigerant circuit), flow / pressure loss (water side), electrical (compressor start, phase), outdoor unit (fan, defrost, sensor).
- One code + a pattern beats one code + no pattern: recurring codes on the same day/hour point to a real cause; a one-off event usually doesn't.
The universal first-response checklist (do before calling anyone)
- Read the exact code from the display/log (not a photo of the panel) and write down date, time, and what happened just before (power cut, storm, frost).
- Check the obvious: outdoor unit free of snow/leaves/ice, breaker and RCD not tripped, water pressure gauge in range, filter accessible and not visibly clogged.
- Reset once (power cycle) — many protection events clear themselves. If the code returns within hours, that's information: the cause is real, not a glitch.
- Export or photograph the log — service techs work faster with history than with "it did something last week".
Common codes by subsystem (indicative, cross-brand patterns)
Scroll the table sideways →
| Subsystem | Typical pattern | Likely cause | Safe DIY action |
|---|---|---|---|
| High pressure / high discharge | compressor stops, high-pressure alarm, often in warm spells or with a dirty coil | overheating: dirty condenser, refrigerant overcharge (service), weak airflow | clear snow/leaves around the outdoor unit; let it cool; if recurring → service with F-gas certification |
| Low pressure / suction | alarm + reduced output, often in cold or after leaks | refrigerant leak, low charge, sensor fault | don't run it hard; call service — top-up is F-gas-certified work |
| Water-side pressure / flow loss | flow error, pressure drop alarm | air in the system, clogged magnetic filter, micro-leak (see pressure drop guide) | bleed the system, check gauge, clean filter — often solves it |
| Compressor start failure | "no start" after reset attempts | electrical (capacitor, contactor), board, or protection lockout after too many retries | power cycle once; if it repeats → service — do not repeatedly force starts |
| Fan / defrost errors | winter only, recurring cycles | fan motor/impeller blockage, defrost sensor drift | check fan spins freely and is clear of ice; recurring → service (see winter behavior guide) |
When it IS worth calling service immediately
- Compressor locked out after repeated resets (further forcing can damage it).
- Refrigerant smell or visible frost/oil on the outdoor unit — assume a leak until it is repaired.
- Any electrical burning smell or tripped RCD — power off, service only.
Affiliate slot: none planned for this article (diagnostic/informational).
Sources
- Typical error code tables of major ASHP brands (Daikin Altherma, Vaillant aroTHERM, Samsung EHS); as of October 1, 2026 — patterns, not exhaustive lists.
- F-gas Regulation (EU) 2024/573 — leak handling and certification requirements; as of October 1, 2026.