Grant A1 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A1 fault code mean?
The A1 fault code on the Grant Aerona and Aerona³ heat pump signals a discharge temperature error. This means the control system has detected an abnormal temperature reading at the compressor discharge point — the hot-gas outlet on the compressor where refrigerant leaves at high pressure and temperature. The unit will typically stop operating to protect the compressor from damage. When this code appears, it shows on the Aerona remote controller (accompanied by a flashing red LED and a warning icon) and simultaneously as a two-digit code on the LED display of the Terminal PCB 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
- Faulty discharge temperature sensor (thermistor) Common
The thermistor that monitors the temperature of the refrigerant leaving the compressor can fail or drift out of its calibrated range. When it sends an implausible reading to the main PCB, the controller triggers the A1 code as a protective measure. This is the single most frequent cause and is usually confirmed by checking the sensor's electrical resistance with a multimeter against the manufacturer's reference values.
- Refrigerant leak in the sealed circuit Common
A loss of refrigerant charge causes abnormal operating pressures throughout the system. With less refrigerant circulating, the remaining gas can superheat excessively at the compressor discharge, pushing temperatures beyond the safe threshold and triggering A1. A refrigerant leak will also reduce heating output noticeably before lockout occurs. Only an F-Gas registered engineer may legally handle or recharge refrigerant in the UK.
- Restricted airflow or blocked evaporator coil Sometimes
If the outdoor unit's airflow is significantly reduced — by accumulated debris, compacted snow and ice, or vegetation growing too close — the heat pump operates under strain. The compressor works harder, discharge temperatures rise, and the A1 safety threshold may be reached. This cause is more likely after prolonged cold snaps or if the unit has not been serviced recently.
- Electrical supply instability or voltage drop Rare
Significant drops or fluctuations in the incoming mains voltage can cause the compressor to operate outside its normal parameters, which can in turn produce elevated discharge temperatures and trigger the A1 code. This is less common but worth noting if the fault appears during periods of high electrical demand or if other appliances have shown sensitivity to supply quality.
How to fix it
- Switch the heat pump off at the outdoor isolator and check the consumer unit for any tripped MCB DIY safe
Before anything else, turn the heat pump off using the external electrical isolator next to the outdoor unit. Go to your consumer unit (fuse board) and check whether any circuit breakers have tripped. If one has tripped, reset it once. If it trips again immediately, leave it off and call an engineer — do not keep resetting it.
- Clear any debris, leaves, or snow from around the outdoor unit DIY safe
Walk around the outdoor unit and check that nothing is blocking the air intake or discharge grille. Remove any leaves, dirt, or garden debris by hand. If snow has built up around or over the unit, carefully clear it away. Ensure there is at least one metre of clear space around the unit on all sides. Do not use sharp tools near the fins of the coil.
- Check the system pressure on the heating circuit DIY safe
Look at the pressure gauge on your indoor unit or cylinder setup. The system pressure when cold should typically read between 1.0 and 1.5 bar (check your system's specific requirement). If it is below 1.0 bar, top up via the filling loop following the instructions in your user manual. This step won't directly fix an A1 fault but rules out a secondary low-pressure issue before calling an engineer.
- Do not attempt to reset the fault code yourself unless advised by your engineer DIY safe
Grant advise that fault codes should only be reset once the underlying cause has been identified and fixed. Repeatedly resetting without resolving the fault risks compressor damage. If your engineer asks you to reset remotely, it can be done by pressing the – and + buttons on the Aerona remote controller simultaneously for three seconds, or by switching the isolator off and on again — but only do this when instructed.
- Call an MCS-certified heat pump engineer to diagnose and repair the fault heat pump engineer
The A1 code nearly always requires professional diagnosis. An engineer will check the discharge sensor resistance against reference values, inspect the refrigerant circuit pressures, and assess the compressor's condition. If a refrigerant leak is found, an F-Gas registered engineer must carry out the recovery, leak repair, and recharge — this is a legal requirement in the UK. Do not delay: running the unit repeatedly with an unresolved A1 fault can escalate a minor sensor fault into compressor damage.
Parts you may need
- Discharge temperature thermistor (sensor) · from £45
- Terminal PCB (control board) · from £280
- Refrigerant recharge (R32, labour and gas) · from £250
- Compressor (unit-size dependent) · from £950
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–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the A1 fault on my Grant Aerona myself?
Grant's own guidance advises against resetting the A1 code without first fixing the underlying fault. Clearing the code without addressing the cause can lead to repeated lockouts or, worse, compressor damage if the unit restarts into an unresolved fault condition. If your engineer asks you to perform a remote reset as part of a diagnostic process, you can press the – and + buttons simultaneously for three seconds on the Aerona remote controller. Otherwise, leave the reset to your service engineer.
Is the Grant Aerona A1 fault dangerous?
The A1 code itself is a protective lockout — the heat pump has shut itself down to prevent further damage, so the immediate risk to the system is contained. However, if the underlying cause is a refrigerant leak, there are environmental and legal obligations: R32 refrigerant must be handled only by an F-Gas registered engineer. There is no direct safety risk to occupants in the home, but you will lose heating and hot water, so acting quickly — especially in cold weather — is important.
How much does it cost to fix a Grant Aerona A1 fault?
For most people, the A1 fault turns out to be either a failed discharge sensor or a refrigerant leak. A sensor replacement including engineer callout and labour typically falls in the region of £150–£350. A refrigerant leak repair with recharge usually costs £300–£500 depending on the severity of the leak and the amount of refrigerant required. In a small number of cases, if the fault has been left unresolved and the compressor has been damaged as a result, costs can rise significantly — compressor replacement on an Aerona³ can run to £1,500 or more. Getting the unit diagnosed promptly gives you the best chance of a lower repair bill.
Why does my Grant Aerona keep showing A1 after being reset?
If the A1 code keeps returning after a reset, the root cause has not been resolved. The most likely explanation is either a discharge sensor that is failing intermittently (and will eventually fail completely), or a slow refrigerant leak that is progressively worsening. Each time the unit restarts and hits the same fault threshold, it locks out again. An engineer needs to carry out a proper diagnostic — including measuring sensor resistance and checking system pressures — to find and fix the actual fault rather than just clearing the code.