Grant C1 Fault Code: Causes, Fixes & Repair Costs
What does the Grant C1 fault code mean?
The C1 code on the Grant Aerona³ indicates a fault detected in the compressor discharge temperature sensor — the thermistor that monitors the temperature of refrigerant as it leaves the compressor. The heat pump's PCB relies on this sensor as a safety check; if the reading falls outside expected parameters (whether due to a sensor failure or a genuine high-discharge condition), the unit will trigger a protective shutdown to prevent compressor 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 discharge temperature thermistor Common
The sensor itself has developed an open or short circuit, causing it to report an out-of-range reading to the main PCB. This is the single most common root cause of a C1 fault and does not necessarily mean anything is wrong with the refrigerant circuit itself.
- Refrigerant undercharge or leak Sometimes
Low refrigerant causes the compressor to work harder than designed, driving discharge temperatures to abnormally high levels. The sensor may be reading correctly and the C1 code is a genuine thermal protection response. Signs include reduced heating output over time before the fault appeared.
- Wiring or connector fault Sometimes
Corrosion, a loose terminal, or physical damage to the wiring loom between the discharge sensor and the main PCB can produce a false or intermittent signal that the controller interprets as a sensor fault.
- Blockage in the refrigerant circuit Sometimes
A partially blocked expansion valve or filter drier can restrict refrigerant flow and elevate discharge temperatures, triggering the same protective response as a refrigerant undercharge.
- Main PCB fault Rare
In rare cases the circuit board itself misinterprets the sensor signal even though the sensor and wiring are both healthy. This is typically only confirmed after all other causes have been ruled out.
How to fix it
- Do not attempt to reset the fault code yourself heat pump engineer
Grant UK advises that fault codes on the Aerona³ should be reset only by a qualified heat pump engineer. Resetting without addressing the underlying cause can mask a condition that may damage the compressor. Leave the unit in its shutdown state until an engineer has diagnosed the fault.
- Check the outdoor unit is clear of obstructions DIY safe
Walk around the outdoor unit and remove any leaves, debris, or compacted snow from the intake and discharge grilles. Restricted airflow can contribute to elevated operating temperatures. Do not use tools to force anything from within the fan guard.
- Check that the system pressure is within the normal operating range DIY safe
Look at the system pressure gauge on your buffer vessel or heating circuit. The pressure should typically sit between 1.0 and 1.5 bar when cold. If it has dropped below 1.0 bar, top up via the filling loop following your system manual. Low system pressure is not a direct cause of C1 but is worth correcting before an engineer visits.
- Check the electrical supply and isolator DIY safe
Confirm that the outdoor unit's dedicated isolator switch is in the on position and that no MCB on your consumer unit has tripped. If a breaker has tripped, reset it once — if it trips again, do not reset it a second time and contact an engineer.
- Engineer: test the discharge sensor resistance heat pump engineer
An MCS-certified engineer will use a calibrated multimeter to measure the thermistor resistance and compare it against the expected values in the Grant Aerona³ fault-finding table. A reading outside tolerance confirms the sensor has failed and needs replacing with a genuine Grant part.
- Engineer: inspect wiring and connectors on the sensor loom heat pump engineer
The engineer will check for corrosion, pin damage, or chafed insulation on the wiring between the discharge sensor and the main PCB. Even a high-resistance connection can produce a false fault reading.
- Engineer: assess the refrigerant circuit heat pump engineer
If the sensor and wiring are healthy, the engineer will assess the refrigerant charge and inspect the expansion valve and filter drier for blockages. Any refrigerant recovery, leak repair, or recharging work must be carried out by an F-Gas registered engineer.
- Engineer: replace the discharge temperature sensor if confirmed faulty heat pump engineer
A confirmed failed thermistor is replaced with a genuine Grant-approved part. The engineer will then clear the fault code from the PCB — either via the remote controller's reset procedure or, for codes that require a hard reset, via the isolator or the Terminal PCB Reset SW button — before testing the unit through a full heating cycle.
- Engineer: check or replace the main PCB if the fault persists heat pump engineer
If the sensor, wiring, and refrigerant circuit all check out correctly but C1 returns, the main PCB is the remaining suspect. PCB replacement should only be carried out by a qualified engineer using a Grant-compatible board to preserve any remaining warranty.
- Contact an MCS-certified Grant Aerona³ engineer DIY safe
If you have not already done so, arrange a visit from a Grant-authorised or MCS-certified heat pump engineer. Using an unauthorised engineer may void your warranty. Grant UK's own ServicePlan network can provide authorised engineers. For any work on the refrigerant circuit, confirm the engineer also holds F-Gas registration.
Parts you may need
- Discharge temperature thermistor / sensor · from £35
- Sensor wiring loom / connector · from £25
- 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–£400, depending on the underlying cause.
Frequently asked questions
Can I reset the C1 fault code myself on a Grant Aerona³?
Grant UK advises against homeowners resetting fault codes on the Aerona³ without an engineer diagnosing the cause first. Some codes clear automatically once the fault is resolved, while others must be manually reset by an engineer via the remote controller or Terminal PCB. Resetting a C1 without addressing the underlying sensor or refrigerant issue risks further compressor damage.
How much does it cost to fix a C1 fault on a Grant Aerona³?
For the most common scenario — a failed discharge thermistor — you would typically pay in the region of £150 to £350 including call-out, diagnosis, part, and labour. If the root cause turns out to be a refrigerant undercharge or leak, add £200–£400 for recovery, repair, and recharge. A PCB replacement sits above this range at roughly £400–£600 including labour. In the rare event the compressor itself has been damaged by prolonged high-discharge operation, costs can be considerably higher — this is worth mentioning to your engineer if the unit has been running with low refrigerant for some time.
Is the C1 fault dangerous — do I need to stop using the heat pump immediately?
The Aerona³ will generally lock out automatically when C1 appears, so the compressor is already protected. You should leave the unit in shutdown and not attempt to force it back on. While there is no risk of fire or gas leak with a heat pump, running a compressor with a faulty discharge sensor or low refrigerant can cause irreversible compressor damage, significantly increasing repair costs. Call an engineer promptly.
Does a C1 fault always mean I have a refrigerant leak?
Not necessarily. A refrigerant leak is one cause of C1, but the most common cause is simply a failed discharge temperature thermistor — the sensor has developed a fault internally rather than the refrigerant circuit itself. An engineer will test the sensor resistance first before moving on to assess the refrigerant charge, so do not assume you need a costly recharge until a proper diagnosis has been carried out.