Grant A3 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A3 fault code mean?
The A3 fault code on the Grant Aerona³ heat pump indicates a Current Transformer (CT) disconnection fault. The CT sensor is a small monitoring device on the main PCB that continuously measures the electrical current drawn by the compressor. When the PCB loses the CT signal — either because the sensor has become disconnected, the wiring has failed, or the PCB itself is unable to process that signal — the system raises the A3 alarm as a protective measure to prevent the compressor from operating outside safe current limits. The unit will typically stop operating and the fault code will appear on both the Aerona Remote Controller display and the LED display on the unit's Terminal PCB, accompanied by a flashing red LED on the controller's on/off switch.
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
- Disconnected or damaged CT sensor wiring Common
The most frequent cause of the A3 fault. The wiring loom connecting the CT sensor to the main PCB can work loose over time due to vibration, or may corrode in the damp environment inside the outdoor unit. A partial disconnection is enough for the PCB to lose confidence in the CT reading and trigger the alarm.
- Failed CT sensor Sometimes
The CT sensor itself can degrade or fail, particularly after several years of operation or following a power-supply disturbance. A failed sensor produces an out-of-range or absent signal that the PCB interprets as a disconnection fault.
- Faulty main PCB Sometimes
If the PCB's CT input circuit is damaged — for example following a nearby lightning strike or a transient voltage spike — it may incorrectly report a CT fault even when the sensor and its wiring are perfectly intact. Visible signs can include burn marks or moisture ingress on the board.
- Faulty reactor (inductor) causing compressor overload Rare
The A3 code is also associated with unreasonable compressor loading. A defective reactor can cause abnormal current draw that stresses the CT monitoring circuit and contributes to this fault code being triggered.
How to fix it
- Check the mains supply and the unit's isolator switch DIY safe
Confirm that the dedicated isolator near the outdoor unit is in the ON position and that no MCB or RCD in the consumer unit has tripped. A power interruption or transient can sometimes leave the controller displaying a latched fault. Restore power if it has been lost.
- Clear any debris from around the outdoor unit DIY safe
Ensure that leaves, snow, or other material are not blocking the outdoor unit's air intake or discharge grille. While not a direct cause of A3, restricted airflow can increase compressor load and contribute to electrical faults over time.
- Attempt a manual reset from the Aerona Remote Controller DIY safe
Some fault codes on the Aerona³ will clear automatically once the underlying issue has resolved. If the fault persists after restoring power, you may attempt a single manual reset from the remote controller. Grant advises that manual resets should ideally be carried out by the installer or service engineer; if the fault returns immediately, do not continue resetting — the underlying fault needs professional investigation.
- Have an engineer inspect CT sensor wiring and connections heat pump engineer
An MCS-certified engineer will open the outdoor unit, visually inspect the CT sensor wiring for loose terminals, corrosion, or breaks, and re-seat or replace any suspect connections. This is the most commonly needed repair for an A3 fault and often restores normal operation quickly.
- Have the CT sensor tested and replaced if necessary heat pump engineer
Using appropriate test equipment, the engineer will verify the CT sensor's output. If the sensor is producing an incorrect or absent signal, it will need to be replaced with a Grant-approved part.
- Have the main PCB and reactor/inductor inspected heat pump engineer
If the wiring and CT sensor check out correctly, the engineer will inspect the main PCB for signs of damage such as burned tracks, swollen capacitors, or moisture ingress. The reactor (inductor) should also be tested; Grant's own fault-finding guidance notes that a faulty reactor should be replaced. PCB replacement must be carried out by a qualified engineer.
- Contact an MCS-certified heat pump engineer if the fault persists heat pump engineer
If the steps above do not identify and clear the fault, arrange a visit from an MCS-certified heat pump engineer with Grant Aerona³ experience. Any work involving the refrigerant circuit or compressor must be carried out by an F-Gas registered engineer. Do not continue to attempt manual resets without professional guidance.
Parts you may need
- CT (Current Transformer) sensor · from £35
- CT sensor wiring loom / harness · from £45
- Grant Aerona³ main PCB · from £420
- Reactor / inductor (compressor circuit) · from £95
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
Can I reset the A3 fault code on my Grant Aerona³ myself?
Grant advises that fault code resets on the Aerona Remote Controller should ideally be carried out by the heat pump installer or a service engineer. While a single reset attempt is unlikely to cause harm, some faults require manual intervention that only a qualified engineer can perform safely. If the fault returns immediately after a reset, stop resetting and book an engineer — repeated resets without resolving the underlying problem can cause additional damage.
How serious is the A3 fault — will it damage my heat pump?
The A3 fault is a protective lockout designed to prevent the compressor from being damaged by running outside safe current limits. As long as the unit is not forced to run while the fault is active, no further damage should occur. The most common cause — a loose CT sensor wire — is a straightforward repair. However, leaving it unattended for an extended period means your home will be without heating or hot water, so prompt investigation is advisable.
What does the repair typically cost for an A3 fault on a Grant Aerona³?
For the most common cause — a wiring fault or CT sensor replacement — most UK homeowners pay in the region of £150–£450 including call-out, labour, and parts. Main PCB replacement is a less common outcome and typically costs £500–£900. Compressor-related repairs are rare for an A3 code but, when required, can cost considerably more; these are best discussed with your engineer once a full diagnosis has been carried out.
Will the A3 fault affect my Grant warranty?
Grant's warranty conditions require the unit to be serviced annually by an authorised engineer. If your service history is up to date, a component fault such as a CT sensor failure should be assessed against your warranty terms. Contact Grant UK directly or your installing engineer with your unit's serial number to check your current warranty position before authorising any paid repairs.