L5 on a Grant heat pump: Humidity sensor error
Short answer
On a Grant heat pump, L5 means: Humidity sensor error. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manual says: Humidity sensor error.
Part involved: humidity sensor.
When it shows: Detected by heat pump/controller PCB; error reset type printed as 'Auto'.
Likely causes:
- Humidity sensor faulty.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: Auto: resets automatically once conditions return to normal (if the heat pump has stopped it may need a manual reset).
Manual reset: Engineer: press - and + on the remote controller together for 3 s, or press Reset SW on the Terminal PCB.
What you can check
- Note the exact code shown on the controller display and tell your installer or Grant-trained engineer.
- Grant advises homeowners NOT to reset Aerona3 fault codes themselves - leave the reset to the engineer.
What the engineer will check
- Check the resistance/voltage by tester; replace if faulty.
Who to ask for: Grant-trained (G1) heat pump engineer.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- Cooling mode confusion / blowing cold on the Aerona³ R32: 1 report
Sources
- Grant Aerona³ Air to Water High Efficiency Heat Pump Range - Installation and Servicing Instructions (UK DOC 0136) ↗ · Rev 2.4 · page 57 (DOC 0154: 56) · Reviewed 23 September 2026
- Grant UK Aerona³ Air to Water High Efficiency Heat Pump Range - Installation and Servicing Instructions (UK DOC 0109) ↗ · Rev 1.1 · page 56 (IRL DOC 0001: s11.4-11.5, printed pp55-57) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026