Grant C5 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C5 fault code mean?
C5 on a Grant Aerona³ heat pump reports a main PCB sensor error: a sensor input on the main (inverter) circuit board itself — not one of the external clamp-on thermistors — has failed its checks. The main board carries its own internal measurement points, including the temperature sensing that protects its power electronics, and when one of these on-board channels fails there is nothing to unplug and swap: the fault is in the board. Grant's documented remedy reflects that directly — the main PCB should be replaced. A single power-cycle to rule out a transient is worthwhile; beyond that, this is a parts-and-labour visit.
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
- On-board sensor failure on the main PCB Common
An internal measurement channel on the inverter board fails — the documented cause, with board replacement the documented remedy.
- Board stress from heat or moisture Sometimes
Power electronics run warm and live outdoors; years of thermal cycling or moisture ingress degrade on-board components until a channel fails.
- Transient after a supply event Sometimes
A spike can upset the board's measurement briefly — the case a single restart genuinely clears.
How to fix it
- Power-cycle once at the external isolator DIY safe
Switch the heat pump off at its external rotary isolator, wait about three minutes for the inverter capacitors to discharge, and switch back on. One controlled restart is legitimate; if the code returns, leave it for an engineer.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
If C5 persists, the engineer will replace the main PCB and recommission the unit — verifying compressor and fan operation and the protection functions before handing back. 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
- Main PCB (inverter board) · from £300
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 £200–£480, depending on the underlying cause.
Frequently asked questions
What does C5 mean on my Grant Aerona³?
A sensor built into the main circuit board has failed — not one of the external temperature probes, but a measurement channel on the board itself. After one power-cycle to rule out a glitch, the documented fix is main PCB replacement by an engineer.
Why can't just the sensor be replaced?
Because it is a component of the board rather than a plug-in probe — board-level component repair is not supported in the field, so the PCB is exchanged as a unit. That is also why C5 costs more than the external sensor codes like A6 or E4.
What will it cost?
Main PCB replacement typically lands in the £200–£480 region fitted, varying with model and region. If the unit is under warranty — Grant's heat pumps carry long warranties when registered and serviced — check that first: this is exactly the kind of repair warranty exists for.