Grant C4 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C4 fault code mean?
The C4 fault code on the Grant Aerona³ heat pump indicates a problem with the outgoing (flow) water temperature sensor — the NTC thermistor that monitors the temperature of water leaving the heat pump. When this sensor reports a reading outside its expected range, the controller flags C4 and may lock out the unit to protect the system. The fault code appears on the Aerona³ Remote Controller display alongside a warning icon and a flashing red LED, and is simultaneously shown on the Terminal PCB LED display inside the outdoor unit.
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
- Failed or degraded NTC thermistor Common
Over time, the outgoing water temperature sensor can drift out of tolerance or fail outright, producing resistance readings the PCB cannot interpret. This is the single most frequent cause of a C4 fault and usually requires straightforward sensor replacement by an engineer.
- Loose or corroded wiring connection Common
A poor connection between the sensor wiring harness and the main PCB can mimic a complete sensor failure. Vibration during normal operation can work terminals loose, and moisture ingress can cause corrosion at the connector, both resulting in an intermittent or persistent C4 alarm.
- Sensor probe physically dislodged from its pocket Sometimes
If the thermistor probe has worked loose from its mounting pocket — due to vibration or a previous service visit — it will read ambient rather than flow temperature and trigger the fault. The fix is to reseat and secure the probe correctly.
- Faulty main PCB Rare
If the sensor and all its wiring test within specification but the C4 code persists after a full power cycle, the main PCB itself may be misreading the sensor signal. PCB failure is the least common cause but must be considered once the sensor and connections have been ruled out.
How to fix it
- Check for an obvious system reset first DIY safe
Before calling an engineer, switch the heat pump off at the external isolator, wait at least three minutes to allow the system electronics to fully discharge, then switch it back on. In some cases a transient sensor error will clear automatically once the heat pump returns to normal operation. If C4 reappears, do not attempt further resets yourself — note down the code and call your installer or an MCS-certified heat pump engineer.
- Inspect the sensor wiring harness and connectors heat pump engineer
An engineer will check the wiring harness running from the outgoing water temperature sensor to the main PCB. They will look for loose terminals, signs of corrosion, and any mechanical damage to the cable. Corroded or loose connectors are cleaned, tightened, or replaced as needed.
- Test the NTC sensor resistance heat pump engineer
Using a calibrated multimeter, the engineer measures the sensor's resistance and compares it against the temperature/resistance table in the Grant Aerona³ installation manual (Fault Finding section). An out-of-tolerance reading confirms the sensor has failed and must be replaced.
- Replace the outgoing water temperature sensor heat pump engineer
If the resistance test confirms a faulty thermistor, the engineer replaces it with the correct OEM-specified NTC sensor. The probe is seated properly in its pocket and all connectors are secured before the system is restarted and retested.
- Investigate the main PCB if the sensor tests correctly heat pump engineer
Should the sensor and wiring both test within specification yet C4 returns after a full power cycle, the main PCB is likely at fault. PCB replacement is a more involved job and typically the most costly outcome of a C4 fault — your engineer will confirm whether repair or replacement is appropriate.
- Arrange a manual fault-code reset and system recommission heat pump engineer
Grant's guidance is clear that a C4 lockout should only be manually reset by a qualified heating engineer or your original installer — not the homeowner. Once the root cause is resolved, the engineer resets the code either via the Aerona³ Remote Controller menu or via the Terminal PCB, then verifies the heat pump returns to normal operation.
- Contact an MCS-certified heat pump engineer if you have not already done so heat pump engineer
If the unit has not recovered after a single power-cycle reset, book a visit from an MCS-certified heat pump engineer. For work involving the refrigerant circuit, ensure the engineer is also F-Gas registered. Using a Grant-accredited engineer also helps protect any remaining warranty — Grant UK's website includes an installer locator tool.
Parts you may need
- Outgoing water NTC temperature sensor (flow thermistor) · from £35
- Sensor wiring harness / connector repair kit · from £20
- Grant Aerona³ main 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–£380, depending on the underlying cause.
Frequently asked questions
Can I reset the Grant Aerona³ C4 fault code myself?
Grant's own guidance advises homeowners not to manually reset fault codes on the Aerona³ Remote Controller. Some codes clear automatically once the underlying fault is resolved, while others require a manual reset by your installer or a qualified engineer. You can try a single power-cycle at the external isolator (off, wait three minutes, back on), but if C4 reappears you should call your engineer rather than repeatedly resetting — doing so without fixing the root cause can mask a developing problem.
How much does it cost to fix a Grant Aerona³ C4 fault in the UK?
For the most common outcome — a failed outgoing water NTC sensor — you are typically looking at £150 to £380 including the engineer's call-out, the replacement part, and labour. If the sensor tests fine and the fault turns out to be the main PCB, costs rise considerably; a PCB replacement can bring the total to £500–£800 or more. Getting a quote before work proceeds is always advisable.
Will my Grant Aerona³ still produce heat with a C4 fault showing?
It depends on the severity. In some cases the heat pump may continue running at reduced capacity while logging C4; in others it will lock out completely and stop heating or providing hot water. If the unit has locked out, your home will lose heating and hot water until the fault is cleared — so it is worth arranging an engineer visit promptly, particularly in colder months.
Can a dirty or poorly maintained system cause the C4 fault?
Indirectly, yes. Heavily contaminated system water or sludge build-up can cause localised temperature stratification around the sensor pocket, producing erratic readings. Using the correct inhibitor concentration and having the system flushed during annual servicing reduces the risk of sensor-related faults. Grant recommends annual servicing of Aerona³ units, which typically includes checking sensor readings and electrical connections — the kind of checks that can catch a deteriorating thermistor before it triggers a lockout.