Grant C3 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C3 fault code mean?
The C3 fault code on the Grant Aerona³ air source heat pump indicates that the unit has detected an abnormally high leaving-water temperature — the temperature of the water exiting the heat pump and entering your heating circuit has exceeded a safe operating threshold. As a protective measure, the heat pump shuts itself down to prevent damage to internal components. A flashing red LED on the remote controller and a warning icon on the LCD display will accompany the code.
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
- Restricted or low water flow through the system Common
If water is not circulating freely around the heating circuit — due to closed isolating valves, an airlock, or insufficient system pressure — heat cannot be transferred away from the heat pump efficiently. This causes the leaving-water temperature to rise rapidly until the unit shuts down for self-protection. This is the most frequent root cause of the C3 fault.
- Blocked inline strainer or system filter Common
Most heat pump installations include an inline strainer or Y-filter to catch debris. If this becomes clogged, it creates a significant restriction to flow, reducing the volume of water passing through the heat exchanger and causing temperatures to spike. Strainers can block gradually over months without any obvious symptoms until a fault code appears.
- Failing or seized circulation pump Sometimes
The system's circulation pump is responsible for moving water continuously around the heating circuit. If the pump is wearing out, running at reduced speed, or has seized, water flow drops sharply and the leaving-water temperature climbs. A pump problem will often also show up as uneven heat distribution around the property before a fault code appears.
- Airflow obstruction around the outdoor unit Sometimes
The outdoor unit relies on unrestricted airflow across the evaporator to operate efficiently. Leaves, garden debris, overgrown vegetation, or snow build-up blocking the air inlet or outlet grilles can impair heat exchange, pushing system temperatures out of balance and contributing to a high leaving-water temperature condition.
- Refrigerant circuit overcharge or fault Rare
An overcharged refrigerant circuit — where the system contains more refrigerant than the manufacturer's specified charge — disrupts the thermodynamic cycle and can cause abnormally high pressures and temperatures on the water side. This is a less common cause but will require an F-Gas registered engineer to diagnose and correct.
- Faulty outgoing water temperature sensor Rare
The heat pump relies on a thermistor to measure the leaving-water temperature. If this sensor has drifted out of calibration, developed a poor connection, or failed outright, it may report a falsely elevated temperature reading and trigger the C3 protection shutdown even when the actual water temperature is within acceptable limits.
How to fix it
- Do not attempt to reset the unit immediately DIY safe
Resetting the heat pump before identifying and resolving the underlying cause will likely result in the C3 fault returning within a short time, and repeated high-temperature shutdowns can cause longer-term stress to the compressor and heat exchanger. Investigate first.
- Check system pressure on the filling loop gauge DIY safe
Low system pressure reduces the volume of water in the circuit, which can cause temperatures to rise quickly. The system pressure should typically sit between 1.0 and 2.0 bar when cold. If it reads below 1.0 bar, top up carefully using the filling loop until pressure is within the normal range, then check again once the system is running.
- Inspect all isolating valves on the heating circuit DIY safe
Walk around the system and check that every isolating valve — on the heat pump itself, on the buffer vessel, and at any zone valves — is fully open. A partially closed valve can dramatically restrict flow without being immediately obvious.
- Clear any debris from around the outdoor unit DIY safe
Check that the air inlet grille and the area around the outdoor unit are free from leaves, grass cuttings, snow, or any other obstruction. Maintain at least the clearances specified in the installation manual. Do not use a pressure washer or insert anything into the fan grille.
- Check and clean the inline strainer or Y-filter heat pump engineer
The inline strainer should be inspected and cleaned by a qualified engineer as part of this investigation, since isolating sections of the system and handling water connections requires competence with pressurised heating systems. An engineer can flush the strainer and check whether restricted flow was the primary cause.
- Have an engineer check the circulation pump operation heat pump engineer
A heat pump engineer can confirm whether the circulation pump is operating at the correct speed and delivering adequate flow. Pump replacement requires draining and refilling part of the system and is not a homeowner task.
- Have an engineer inspect the leaving-water temperature sensor heat pump engineer
An engineer can test the thermistor resistance against the manufacturer's characteristic curve to determine whether the sensor is reading accurately or triggering a false fault. Sensor replacement involves accessing internal components of the heat pump.
- Have an F-Gas registered engineer assess the refrigerant circuit if no other cause is found heat pump engineer
If all water-side checks are clear, the refrigerant circuit may be contributing to the fault. Only an engineer holding an F-Gas qualification is legally permitted to handle, recover, or recharge refrigerant in the UK. Do not attempt to access any refrigerant components yourself.
- Allow an engineer to perform the manual fault reset heat pump engineer
Once the underlying cause has been resolved, the C3 code may need to be manually cleared. On the Aerona³, this can be done via the remote controller (pressing – and + simultaneously for three seconds) or via the Terminal PCB Reset button. Grant advises that fault resets should be carried out by the installer or service engineer, not the homeowner, to ensure the fault has genuinely been resolved first.
- Contact an MCS-certified heat pump engineer if the fault persists or you cannot identify the cause heat pump engineer
If the C3 code returns after basic checks, or you are unable to identify a clear cause, arrange a visit from an MCS-certified heat pump engineer. Bring your installation manual or have the model and serial number to hand. Refrigerant work must be carried out by an F-Gas registered engineer.
Parts you may need
- Leaving-water temperature sensor (thermistor) · from £45
- Inline Y-strainer / system filter · from £35
- System circulation pump (replacement) · from £180
- Grant Aerona³ Terminal PCB · from £220
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–£450, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my Grant Aerona³ heat pump when the C3 fault is showing?
No. The C3 code indicates the heat pump has already shut itself down as a protective measure. Running it repeatedly in this state risks damaging the compressor and heat exchanger, which are far more expensive to repair. Leave the unit off and arrange for an engineer to investigate before restarting.
Can I reset the C3 fault code myself on the Grant Aerona³?
Grant's own guidance is that fault code resets on the Aerona³ should be performed by the installer or a qualified service engineer, not the homeowner. This is because some codes reset automatically once the fault clears, while others require a manual reset — and resetting without fixing the root cause simply delays proper diagnosis. An engineer can use the remote controller or Terminal PCB to reset the code once they are satisfied the fault has been resolved.
How much does it typically cost to fix a C3 fault on a Grant Aerona³?
For the most common causes — a blocked strainer, low pressure, or a faulty temperature sensor — most homeowners pay somewhere between £150 and £450 including the engineer callout, labour, and any parts. Circulation pump replacement typically sits towards the upper end of that range. In rarer cases where the refrigerant circuit is involved or if the compressor has sustained damage from repeated high-temperature shutdowns, costs can rise significantly beyond this — potentially £500 to £900 or more. An engineer will confirm a quote before proceeding with any major work.
Could the C3 code be a false alarm caused by a faulty sensor rather than a real overheating problem?
Yes, this is possible. The leaving-water temperature sensor (a thermistor) can drift out of calibration or develop a poor electrical connection, causing it to report a falsely high reading. However, a faulty sensor is one of the less common causes of C3 — restricted water flow, a blocked strainer, and pump issues are all more likely. An engineer can test the sensor's resistance to confirm whether it is reading correctly before replacing it unnecessarily.