Grant A4 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A4 fault code mean?
A4 on a Grant Aerona³ heat pump is an AC current detection error — the unit's protection against excessive current draw on the incoming mains side has operated. Grant's documented remedies point at two practical checks before anything else: the installation position (a blocked air inlet or outlet makes the unit work abnormally hard, dragging current up) and the power supply itself (confirm a healthy 230V). If both are sound and the code persists after a restart, the fault moves inside — to the measurement circuit on the main PCB or a component genuinely drawing excess current. It is the mains-side sibling of A2, which covers the same supervision on the DC side of the inverter.
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
- Blocked air inlet or outlet Common
Grant's first documented remedy: an obstructed outdoor unit works harder for less heat, pulling current toward the protection threshold — especially in cold snaps with ice or drifted snow.
- Power supply out of specification Sometimes
Low or unstable mains voltage raises current for the same output; the documentation says to verify a proper 230V supply before suspecting hardware.
- Main PCB measurement or component fault Sometimes
With airflow and supply proven, a persisting A4 points at the current-detection circuit or a genuinely overloaded component — engineer territory, often ending in board replacement.
How to fix it
- Check the outdoor unit for obstructions DIY safe
Make sure the air inlet and outlet are clear of leaves, snow, ice or anything stored against the unit — Grant's own remedy notes for several codes start with the installation position and airflow. Clear debris gently by hand, never with tools near the fan.
- Power-cycle once at the external isolator DIY safe
Switch the heat pump off at its external rotary isolator, wait about three minutes for the inverter capacitors to discharge, and switch back on. One controlled restart is legitimate; if the code returns, leave it for an engineer.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will verify the 230V supply under load, measure the actual running current against specification, and inspect the fan, compressor and main PCB — replacing the board if the detection circuit itself is at fault, per Grant's documentation. Work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; your installer or a Grant-accredited (G1) installer is the right call, and Grant UK technical support (01380 736920) can assist with diagnosis.
Parts you may need
- Main PCB (inverter board) · from £300
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 £100–£450, depending on the underlying cause.
Frequently asked questions
What does A4 mean on my Grant Aerona³?
The heat pump detected excessive current on its mains supply side and shut down protectively. Check the outdoor unit's air inlet and outlet are completely clear, then power-cycle once at the isolator. If A4 returns with good airflow, the supply voltage and the main board need professional checking.
Why would a blocked outdoor unit cause an electrical code?
Restricted airflow makes the fan and compressor work much harder for the same heat, and that extra work shows up directly as extra current. Grant's own remedy list starts with the installation position for exactly this reason.
Snow and ice keep setting it off — is that normal?
Frost on the coil is normal (the unit defrosts itself), but drifted snow or ice blocking the inlet or outlet is not — clear it gently and consider whether the unit's position needs better protection from drifting. Persistent trouble is worth raising with your installer.