Grant C6 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C6 fault code mean?
The C6 code on a Grant Aerona³ heat pump indicates that the unit has detected a fan failure or an air pressure switch fault. The air pressure switch monitors whether the outdoor fan is generating sufficient airflow; if the fan stops or slows significantly, the pressure switch trips and C6 is logged. On the Aerona³ system the code appears simultaneously on the LCD of the Aerona Remote Controller (accompanied by a warning icon and a flashing red LED) and on the LED display of the Terminal PCB inside the heat pump itself. This is considered a serious ventilation fault — without adequate airflow across the outdoor coil the heat pump cannot operate safely and will typically lock out to protect the compressor.
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
- Fan motor failure or faulty run capacitor Common
The run capacitor gives the fan motor the electrical kick it needs to start spinning. When the capacitor degrades or fails, the motor may hum but not turn, or may fail to reach the correct speed. This is the single most common cause of a C6 fault and is the first thing an engineer will test.
- Physical blockage around the outdoor unit Common
Leaves, debris, overgrown vegetation or a build-up of compacted snow can restrict airflow through the outdoor unit. The pressure switch detects the reduced throughput and triggers C6 even though the fan motor itself may be fine.
- Seized fan motor bearings Sometimes
Over time, especially in units that have not had regular servicing, the motor bearings can seize. The motor may produce a humming or grinding noise but the fan will not spin freely. Bearing failure requires the motor to be replaced.
- Wiring or connection fault Sometimes
Vibration and weather exposure can loosen terminal connections or damage wiring to the fan motor or air pressure switch. A loose or corroded connection can interrupt the signal and generate a spurious C6 code even when the motor is mechanically sound.
- Faulty air pressure switch Sometimes
The pressure switch itself can fail — either stuck open (always reading low pressure) or with a damaged sensing port. This would cause C6 to appear even when the fan is running correctly.
- Main control PCB fault Rare
If the fan motor, capacitor, wiring and pressure switch all test correctly, attention turns to the main PCB. A failed output driver or relay on the board can prevent the fan from receiving its run signal.
How to fix it
- Check the power supply and any tripped circuit breakers DIY safe
Go to your consumer unit and confirm the MCB or RCBO for the heat pump has not tripped. If it has tripped, reset it once. If it trips again immediately do not attempt a further reset — there is an underlying electrical fault and an engineer is needed.
- Inspect the area around the outdoor unit for blockages DIY safe
Walk around the outdoor unit and look for leaves, debris, compacted snow or ice, overgrown plants, or anything else that could be restricting the air inlet or outlet. Carefully clear any debris by hand. Do not use tools or water jets near the unit. Ensure there is at least the manufacturer-recommended clearance on all sides.
- Observe whether the outdoor fan is spinning DIY safe
Stand safely away from the unit and look through the fan grille to see if the fan blade is rotating when the heat pump is calling for operation. A fan that is not moving at all, or is spinning very slowly, confirms a mechanical or electrical fault with the fan circuit — do not attempt to manually spin or interfere with the blade.
- Attempt a controlled restart via the external isolator DIY safe
Switch the heat pump off at the external isolator (usually a rotary switch on the wall near the outdoor unit). Wait at least 60 seconds, then switch it back on. Allow the unit several minutes to restart. Some C6 faults will auto-reset if the root cause was a transient blockage now cleared. If C6 returns, do not continue cycling the power — repeated restarts with a genuine fan fault can stress the compressor.
- Do not attempt to reset the fault code manually on the Terminal PCB heat pump engineer
Certain Grant fault codes require a manual reset performed from the Terminal PCB inside the heat pump. This is not a homeowner task — opening the unit enclosure and operating internal controls should only be carried out by a qualified heat pump engineer.
- Contact an MCS-certified heat pump engineer to diagnose and repair the fault heat pump engineer
A qualified engineer will test the fan motor windings, run capacitor, pressure switch and associated wiring using appropriate instruments. They will replace any faulty component and perform a controlled reset via the Terminal PCB or Aerona Remote Controller. If the fault has affected the refrigerant circuit (for example, due to compressor overheating), refrigerant work must be carried out by an F-Gas registered engineer. You can find Grant-accredited local installers through Grant UK's installer search tool.
Parts you may need
- Fan run capacitor · from £25
- Outdoor fan motor (Aerona³ compatible) · from £180
- Air pressure switch · from £45
- Grant Aerona³ main control PCB · from £320
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 £150–£500, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my Grant Aerona³ while the C6 code is showing?
No — the heat pump will typically lock out when C6 is active, meaning it will not produce heat until the fault is resolved. Even if it does manage to run intermittently, operating without proper airflow across the outdoor coil forces the compressor to work under stress, which can cause overheating and ultimately lead to compressor failure. A compressor replacement is significantly more expensive (often £1,500–£3,000 or more including labour), so it is far better to address C6 promptly.
Can I reset the C6 fault code myself?
A simple power cycle via the external isolator is the only reset a homeowner should attempt. Some C6 faults — particularly those caused by a temporary blockage that you have now cleared — will clear themselves once normal airflow is restored. However, if C6 returns after a single power cycle, the underlying fault needs professional diagnosis. Manual resets from the Terminal PCB inside the unit should only be performed by a qualified engineer.
How much does it typically cost to fix a Grant C6 fault in the UK?
For the most common causes, most UK homeowners pay somewhere between £150 and £500. A capacitor swap is at the lower end (roughly £100–£250 including the call-out and labour), while a full fan motor replacement usually falls in the £300–£500 range. If the main PCB turns out to be the culprit, costs can rise to £600–£900 or beyond, but PCB failure is a less frequent cause of C6. Check whether your unit is still covered by a Grant warranty or ServicePlan, as parts costs may be covered.
How can I prevent the C6 fault from recurring?
Annual servicing by an MCS-certified engineer is the most effective preventive measure. During a service the engineer will check the fan motor, test the capacitor, inspect wiring connections and clear any debris from the unit — catching early signs of wear before they develop into a lockout fault. Keeping the area around the outdoor unit clear of vegetation and debris throughout the year also significantly reduces the chance of an airflow-related C6.