Grant L0 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L0 fault code mean?
L0 on a Grant Aerona³ heat pump is an EEPROM error on the controller: the non-volatile memory holding the controller board's configuration and stored settings has failed its integrity check. A controller that cannot trust its stored parameters cannot safely run a compressor, defrost logic and heating circuit, so the unit stops. Grant's documentation lists the remedy plainly — the controller PCB should be replaced when the error persists — with a power-off reset the documented first response, since a corrupted-but-recoverable memory state occasionally clears on a genuine cold restart. If L0 returns, it is a board replacement, followed by reconfiguration for your installation.
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
- EEPROM corruption after a power event Common
A spike or brownout during a memory write corrupts stored data; sometimes a full power-off restart recovers it, otherwise the board is replaced.
- Controller board memory failure Sometimes
The memory device itself failing brings L0 back on every start — the documented board-replacement case.
- Board degradation from environment Rare
Moisture or thermal stress around the controller can corrupt memory behaviour before anything else visibly fails.
How to fix it
- Power off fully, wait three minutes, power on DIY safe
Grant's documented reset for this code is a power-off restart. Switch off at the external isolator, wait a full three minutes, and switch back on — a genuine cold start is the only kind that counts.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
If L0 returns, the controller PCB is replaced per the documentation and reconfigured for your system — heating curves, cylinder settings and any zone arrangements — then proven through a full heating and hot water cycle. 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
- Controller PCB · from £200
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 £180–£420, depending on the underlying cause.
Frequently asked questions
What does L0 mean on my Grant Aerona³?
The controller board's stored memory failed its integrity check, so the unit stopped — it cannot safely run on settings it does not trust. Try one full power-off restart (three minutes off); if L0 returns, the controller PCB needs replacing and setting up again for your system.
Will my settings survive the repair?
The new board arrives blank, so the engineer reconfigures it — heating curve, hot water schedule, zones. If you know your installer set specific weather-compensation values, mention it; otherwise commissioning defaults plus a tidy-up visit get you back to normal.
Is this covered by warranty?
If the unit is registered and has its annual services, very likely — controller boards are exactly what manufacturer warranties are for. Check your registration before paying for parts.