Grant L5 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L5 fault code mean?
L5 on a Grant Aerona³ heat pump reports a humidity sensor error: a humidity measurement the controller has been configured to use — typically for cooling-mode dew-point protection, where the system must not chill surfaces below the temperature at which condensation forms — has failed or is reading implausibly. Grant's documented diagnosis is a resistance check with a tester, and the remedy is replacement of the faulty sensor. On heating-only installations without a humidity sensor fitted, an L5 can instead point to a configuration mismatch — a control setting expecting a sensor that is not there — which is a settings correction rather than a parts job.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Failed humidity sensor Common
The sensor element drifts or fails — Grant's documented check is its resistance measured with a tester, with replacement the remedy.
- Wiring or connector fault Sometimes
A disturbed lead or corroded connector between sensor and controller reproduces the failure; proven before parts are fitted.
- Configuration expecting a sensor that is not fitted Sometimes
On installations without humidity sensing, a control parameter set as though one exists produces the code — corrected in settings, no parts needed.
How to fix it
- Note whether your system uses cooling DIY safe
Humidity sensing chiefly matters for cooling-mode condensation protection. Knowing whether your installation heats only, or also cools, helps the engineer decide between a failed sensor and a configuration mismatch.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will check the humidity sensor's resistance and wiring where one is fitted and replace it if faulty — or correct the controller configuration if the code stems from a setting expecting a sensor the installation does not have. Work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; your installer or a Grant-accredited (G1) installer is the right call, and Grant UK technical support (01380 736920) can assist with diagnosis.
Parts you may need
- Humidity sensor · from £40
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£260, depending on the underlying cause.
Frequently asked questions
What does L5 mean on my Grant Aerona³?
A humidity reading the controller expects has failed or gone implausible. Where a humidity sensor is fitted — mainly on systems that also cool — it is tested and replaced; on heating-only systems the code can mean a configuration expecting a sensor that is not there, which is a settings fix.
Why does a heat pump care about humidity?
In cooling mode, chilled surfaces below the air's dew point stream with condensation. The humidity sensor lets the controller keep temperatures safely above that line — so when the reading fails, cooling is restricted to be safe.
Will my heating still run with L5 showing?
Often yes — the restriction centres on the functions that need the humidity reading. But do not leave any standing fault code unexplained: have it resolved at the next visit so it is not masking something else.