Grant A6 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A6 fault code mean?
A6 on a Grant Aerona³ heat pump reports a suction temperature sensor error: the thermistor measuring refrigerant temperature on the suction line — the pipe returning cool refrigerant gas to the compressor — has failed or is reading implausibly. The suction reading matters more than its obscurity suggests: the controller uses it to manage superheat, ensuring the refrigerant entering the compressor is fully gaseous, because liquid refrigerant reaching a compressor damages it. With the reading untrustworthy, the unit shuts down rather than gamble. Grant's documented remedy is simple: if the sensor is faulty, replace it — a resistance check against the table settles the diagnosis.
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 suction temperature thermistor Common
The sensor lives clamped to a cold, condensation-prone pipe outdoors and fails open, short or drifted — the documented cause, confirmed with a resistance check.
- Wiring or connector corrosion Sometimes
Outdoor connectors weather; corrosion or water ingress mimics sensor failure and is checked before parts are fitted.
- Sensor dislodged from the pipe Sometimes
A clip-on sensor that has come away from the suction line reads air temperature instead — plausible numbers, wrong physics.
How to fix it
- 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 measure the suction sensor's resistance against the temperature table, inspect its wiring, connector and mounting on the suction line, and replace the sensor if faulty — Grant's documented remedy. 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
- Suction temperature sensor (thermistor) · from £30
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–£280, depending on the underlying cause.
Frequently asked questions
What does A6 mean on my Grant Aerona³?
The sensor measuring refrigerant temperature on the compressor's suction line has failed, so the unit shut down — that reading protects the compressor from ingesting liquid refrigerant. The fix is a confirmed-and-replaced sensor; one of the cheaper heat pump repairs.
Why does one small sensor stop the whole heat pump?
Because it guards the compressor, the unit's most expensive component. Running blind on suction temperature risks liquid slugging the compressor — a far worse bill than a sensor swap and a call-out.
Do I need an F-Gas engineer for a sensor?
The sensor itself clamps to the outside of the pipe, so the sealed circuit is not opened. In practice you want a heat pump engineer regardless — they will verify superheat behaves correctly after the swap, and your installer or a Grant G1 installer is the natural choice.