Grant C7 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C7 fault code mean?
C7 on a Grant Aerona³ heat pump is a controller PCB serial error: the digital communication link between the unit's circuit boards — the controller board and the main inverter board talking over a serial connection — has failed. Grant's documentation attributes it to miswiring or poor contact between the PCBs, with controller board replacement the fallback when the link itself is proven and the fault persists. In practice that makes C7 one of the more hopeful-looking board codes: a reseated connector or corrected wiring often restores communication without any parts at all, and the engineer works through exactly that order — connections first, board second.
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
- Poor contact on the inter-board connection Common
Vibration and thermal cycling loosen or oxidise the serial connector between the boards — Grant's documented first cause, and the cheapest fix on the list.
- Miswiring after service or installation work Sometimes
If the code appeared after work on the unit, a lead re-fitted incorrectly between the PCBs is the prime suspect — the documentation names miswiring explicitly.
- Failed controller PCB Sometimes
With the wiring proven, a persisting C7 means the controller board's communication stage has failed and the board is replaced — the documented remedy.
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.
- Note any recent work on the unit DIY safe
If an engineer was recently inside the unit for any reason, say so when booking — a disturbed inter-board connection is then the likely story and a quick fix.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will inspect and reseat the serial connections between the controller and main PCBs, correct any wiring errors, and replace the controller PCB only if communication still fails with the link proven — Grant's documented order of attack. 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
- Inter-board wiring/connectors · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
What does C7 mean on my Grant Aerona³?
The unit's circuit boards have stopped talking to each other over their serial link. Grant's documentation points first at wiring and connector contact between the boards — often fixed by reseating a connection — with controller board replacement only if the link is proven and the fault remains.
Is this an expensive fault?
It has the widest range of the board codes: a reseated connector costs only the visit, while a controller PCB pushes toward £400. The connection checks come first precisely because they so often resolve it.
The code appeared right after a service — coincidence?
Probably not. A serial connector disturbed during other work is a classic C7 trigger. Call the servicing firm back — a reputable one will return to check their own work without quibbling.