Grant C8 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C8 fault code mean?
C8 on a Grant Aerona³ heat pump reports a main PCB error — the inverter board has failed an internal check that does not map to a more specific code. Grant's documented remedy is unusually explicit about the first step: turn off the power supply, wait about three minutes, then power up again. That pause matters — the inverter's capacitors need time to discharge fully so the board genuinely restarts from cold. If C8 returns after a proper three-minute power-cycle, the documentation is equally direct about the second step: the main PCB should be replaced. There is no intermediate tinkering that rescues a board failing its own self-checks.
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
- Main PCB internal fault Common
The inverter board fails its self-check — component degradation from years of thermal cycling in an outdoor unit is the usual path. Board replacement is the documented remedy.
- Transient upset requiring a full discharge Common
A supply event can leave the board in a confused state that only a genuine cold restart clears — hence the documented three-minute wait, not a quick off-on.
- Moisture or connector issues on the board Sometimes
Water ingress or corrosion around the board can trip internal checks; the engineer looks for a cause before fitting the new PCB so it is not sent the same way.
How to fix it
- Power off, wait a full three minutes, power on DIY safe
Switch off at the external isolator and genuinely wait three minutes — Grant's documented procedure — so the inverter capacitors discharge and the board restarts from cold. A hasty ten-second cycle does not count and will mislead you about whether the fault is real.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
If C8 returns after a proper power-cycle, the main PCB is replaced per Grant's documentation, with the engineer checking for moisture or supply causes and recommissioning the unit afterwards. 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 C8 mean on my Grant Aerona³?
The main circuit board has failed an internal check. Grant's own remedy: switch off, wait about three minutes, switch on. If C8 comes back after that genuine cold restart, the main PCB needs replacing — an engineer's job.
Why exactly three minutes?
The inverter stores energy in large capacitors that take a couple of minutes to discharge. Until they do, the board never truly restarts — so a quick off-on can leave the fault state intact and make a recoverable glitch look like a hardware failure.
Is it worth claiming on warranty?
Absolutely check — Grant heat pumps carry substantial warranties when registered and annually serviced, and a main PCB is one of the costlier parts. Have your registration details and service history ready when you call.