Grant C2 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C2 fault code mean?
The C2 code on a Grant Aerona³ heat pump indicates a fault within the compressor protection system, most likely triggered by an out-of-range reading from the outlet (flow) water temperature sensor, or by an overtemperature condition at the heat pump's water outlet. When the unit detects that the leaving water temperature has exceeded safe limits — or that the sensor monitoring it is giving an implausible signal — it shuts down automatically to protect the compressor from damage.
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
- Faulty outlet water temperature sensor (thermistor) Common
The thermistor responsible for monitoring the temperature of water leaving the heat pump has either failed completely or drifted out of its expected resistance range. This causes the control board to see an impossible or out-of-range value and triggers a protective shutdown, even if the actual water temperature is normal.
- Restricted water flow through the system Common
A partially closed isolating valve, a blocked magnetic filter, or a heavily sludged system can drastically reduce the volume of water flowing through the heat exchanger. With less water to absorb the heat, the outlet temperature rises rapidly until the overtemperature protection fires.
- Low system pressure Common
If the system pressure has dropped below around 1 bar, the circulation pump cannot move water effectively. This reduces flow through the heat exchanger and can cause the outlet water temperature to spike, triggering the C2 shutdown.
- Blocked or fouled plate heat exchanger Sometimes
Limescale, magnetite sludge, or other debris coating the internal surfaces of the heat exchanger reduces heat transfer efficiency. The compressor works harder for less result, and the outlet temperature climbs to a point where the safety trip activates.
- Refrigerant circuit undercharge or leak Sometimes
A low refrigerant charge upsets the balance of pressures within the sealed circuit. High-side pressure can become abnormal, leading to overtemperature conditions at the heat exchanger outlet and ultimately triggering the C2 code.
- Circulation pump failure Sometimes
If the system's circulation pump has seized, lost performance, or failed electrically, water flow stops or becomes insufficient. The heat exchanger outlet temperature then rises uncontrollably until the unit locks out.
How to fix it
- Check the system pressure gauge DIY safe
Locate the pressure gauge on your heating system — it is usually on the buffer vessel, the boiler header, or a pipework manifold near the heat pump's indoor unit. The reading should be between 1.0 and 2.0 bar when the system is cold. If it reads below 1 bar, top up the system via the filling loop until it reaches approximately 1.5 bar. Consult your system manual if you are unsure where the filling loop is located.
- Inspect the outdoor unit for airflow obstructions DIY safe
Walk around the outdoor unit and check that the grilles, top fan outlet, and sides are not blocked by leaves, dirt, plant growth, or accumulated debris. Clear away anything within at least 300 mm of the unit. In winter, check that no ice has bridged across the fan guard in a way that the unit cannot clear through its own defrost cycle.
- Confirm all water circuit isolation valves are fully open DIY safe
Trace the pipework between the heat pump and your buffer vessel or heating circuit. Check that every isolating valve — including any that may have been partially closed during a previous service or maintenance visit — is in the fully open position. Even a valve that is only slightly closed can cause a significant drop in flow rate.
- Do not attempt to reset the fault code yourself DIY safe
Grant advises that some Aerona³ fault codes require manual reset by a qualified engineer, and attempting an unauthorised reset can mask the underlying problem or risk further damage. Leave the unit in its locked-out state and contact an MCS-certified installer or service engineer to carry out the reset once the root cause has been identified and resolved.
- Have an engineer test the outlet water temperature sensor heat pump engineer
An MCS-certified engineer will disconnect the thermistor and measure its resistance against the temperature-resistance curve specified in the Aerona³ installation manual. A sensor that reads open-circuit, short-circuit, or significantly outside the expected range at ambient temperature will need to be replaced. This is an electrical component task requiring system isolation.
- Have an engineer check water flow rate and inspect the magnetic filter heat pump engineer
The engineer will measure flow rate through the heat pump and compare it against the manufacturer's minimum specification. The magnetic filter (such as a Grant Mag-One) will be inspected and cleaned if it is carrying a significant sludge load. A badly fouled filter can reduce flow to a fraction of what the heat pump requires.
- Have an engineer assess the refrigerant circuit pressures heat pump engineer
Using a refrigerant manifold gauge set, an F-Gas registered engineer will check the high-side and low-side pressures against the expected values for the prevailing ambient conditions. An undercharge or contaminated refrigerant circuit cannot be safely assessed or remedied by anyone who is not F-Gas registered — this is a legal requirement in the UK.
- Arrange for an MCS-certified heat pump engineer to carry out a full fault diagnosis and reset heat pump engineer
If the checks above have not identified and resolved the cause, contact an MCS-certified heat pump engineer with experience of Grant Aerona³ systems. They can access the full fault history on the controller, carry out component-level testing, and perform the authorised reset procedure (either via the Remote Controller button combination or via the Terminal PCB Reset SW switch) once the system is confirmed safe to restart.
Parts you may need
- Outlet water temperature thermistor (flow sensor) · from £35
- Magnetic system filter (e.g. Grant Mag-One) · from £85
- Domestic circulation pump (replacement) · from £120
- Aerona³ main control PCB · from £280
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
Can I reset the Grant C2 fault code myself?
Grant's own guidance is that fault codes on the Aerona³ should be reset by a qualified installer or service engineer, not the homeowner. Some codes clear automatically once the underlying problem is resolved, but others require a deliberate manual reset using the controller buttons or the Terminal PCB. Resetting before the root cause is found can allow the fault to recur and risks causing further wear to the compressor. Contact an MCS-certified engineer to diagnose and reset the unit.
How much does it typically cost to fix a Grant Aerona³ C2 fault?
For the most common causes — a failed thermistor, a dirty magnetic filter, or low system pressure — you can expect to pay roughly £150 to £350 including call-out, parts and labour. If the circulation pump needs replacing, costs are typically in the £300–£500 range. Refrigerant recharging by an F-Gas registered engineer typically adds £250–£450. In worst-case scenarios where the compressor itself has been damaged through running in a fault condition for a prolonged period, costs can exceed £2,000–£3,000, though this outcome is far less common when the fault is caught and addressed promptly.
Will low system pressure cause a C2 fault on a Grant Aerona³?
Yes, it can. If system pressure drops below around 1 bar, the circulation pump cannot maintain adequate water flow through the heat exchanger. The water that does pass through picks up more heat than it should, causing the outlet temperature to rise to a level that triggers the C2 overtemperature protection. Topping up the system pressure to around 1.5 bar via the filling loop is a safe check a homeowner can carry out before calling an engineer.
How often should my Grant Aerona³ be serviced to prevent faults like C2?
Grant recommends annual servicing by a qualified heat pump engineer. A service typically includes checking refrigerant pressures, testing all temperature sensors, inspecting and cleaning the magnetic filter, verifying system pressure and flow rates, and reviewing the fault log on the controller. Regular servicing is the most effective way to catch developing problems — such as a drifting thermistor or a slowly clogging filter — before they escalate into a full lockout.