Grant A0 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A0 fault code mean?
The A0 fault code on Grant heat pumps indicates a DC voltage error within the inverter's internal DC bus — the electrical circuit that drives the compressor. The controller has detected a DC voltage that is outside acceptable limits, either too high, too low, or unstable. This is a serious electrical fault and the heat pump will typically shut itself down as a protective measure. On Aerona³ systems, the code appears on the remote controller's LCD screen alongside a warning icon with the red LED flashing, and as a two-digit code on the Terminal PCB display on the outdoor unit itself.
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 or degraded reactor (DC line choke) Common
The reactor is an electromagnetic inductor that smooths out voltage fluctuations on the DC bus feeding the inverter. When it degrades or fails, the DC bus voltage becomes unstable and the controller detects an out-of-range reading. This is the most commonly implicated component in an A0 fault and, if faulty, it will need to be replaced by an engineer.
- Main PCB failure Common
The inverter control board is responsible for regulating and monitoring the DC bus voltage. Burnt components, failed capacitors or relay damage on the PCB can prevent it from maintaining or correctly reading DC voltage, triggering the A0 code.
- Unstable or inadequate mains power supply Sometimes
If the incoming mains supply is fluctuating outside normal limits, or a supply fuse (such as CF1, CF3, CF4, CF6 or CF7 depending on model) has blown, the inverter DC bus may see voltage spikes or drops sufficient to trigger the fault. Poor wiring connections at the isolator or consumer unit can also contribute.
- Internal circulating pump drawing abnormal current Sometimes
The DC-controlled internal circulating pump, if it is seizing, obstructed or failing, can draw irregular current that disturbs the DC bus and causes the inverter to register a voltage error.
How to fix it
- Switch the heat pump off at the external isolator and wait 60 seconds DIY safe
Cutting power completely allows any transient voltage spike to dissipate. Wait a full minute before restoring power. This is the only safe homeowner reset method for this fault code — do not attempt to reset through the controller menus yourself, as the A0 code should be cleared by a qualified engineer once the root cause has been identified.
- Check the consumer unit or dedicated MCB for tripped breakers DIY safe
Go to your electrical consumer unit and check whether the MCB supplying the heat pump has tripped. If it has tripped, reset it once. If it trips again immediately, do not reset it further — this points to an ongoing electrical fault and an engineer is needed urgently.
- Restore power via the isolator and observe whether the fault clears DIY safe
Switch the isolator back on and watch the Aerona³ remote controller. If the A0 code disappears and the heat pump returns to normal operation, monitor it over the next hour. If the code reappears, the underlying fault is persistent and requires engineer investigation.
- Engineer to verify the incoming mains supply voltage and inspect supply fuses heat pump engineer
An MCS-certified engineer should use a multimeter to confirm the supply voltage is within the rated tolerance and check all relevant supply fuses on the unit (CF1, CF3, CF4, CF6, CF7 as applicable to the model). A blown fuse or out-of-tolerance supply must be rectified before proceeding.
- Engineer to test the reactor (DC choke) for continuity and condition heat pump engineer
The engineer should inspect the reactor for visible damage and test continuity. A failed reactor will need to be replaced with a manufacturer-approved part. This is the most commonly implicated component in this fault code.
- Engineer to inspect the main PCB for signs of electrical damage heat pump engineer
The inverter control board should be visually inspected for burnt tracks, swollen capacitors or damaged relays, and DC voltage output tested. A faulty PCB will require replacement — this should only be carried out by a qualified engineer using a genuine Grant replacement part.
- Engineer to check the internal circulating pump operation heat pump engineer
The engineer should confirm the internal circulating pump is receiving correct power, running freely and not drawing excessive current. A failing or seized pump may need to be replaced.
- Engineer to carry out a controlled fault reset and test the system heat pump engineer
Once the root cause has been identified and rectified, the engineer should perform a manual fault code reset via the remote controller or Terminal PCB, then run the system through a full heating cycle to confirm normal operation and stable DC bus voltage before leaving the site.
- If the fault persists or the root cause cannot be resolved on-site, contact Grant UK support or find an accredited installer via grantuk.com/support heat pump engineer
Grant maintains a network of accredited installers. For complex electrical or inverter faults that cannot be resolved through component replacement, specialist support from Grant's technical team may be required. Note that any work involving the refrigerant circuit must be carried out by an F-Gas registered engineer.
Parts you may need
- DC reactor / line choke (Grant Aerona³ compatible) · from £120
- Main inverter PCB (Grant Aerona³ / HPiD series) · from £380
- Internal circulating pump · from £160
- Supply fuse set (CF1/CF3/CF4/CF6/CF7) · from £18
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 £200–£600, depending on the underlying cause.
Frequently asked questions
Can I reset the Grant A0 fault code myself?
You can try switching the heat pump off at the external isolator, waiting 60 seconds and restoring power — this is the safest homeowner reset method. If the code clears and the unit runs normally, monitor it closely. However, Grant advise that manual fault code resets through the controller menus should be carried out by a qualified installer or engineer, not by the homeowner. If the A0 code returns, do not keep resetting it; the underlying electrical fault needs professional diagnosis.
How serious is the A0 fault code on a Grant heat pump?
It is a serious fault. The A0 code means the inverter's DC bus has detected a voltage problem, which causes the heat pump to shut down to protect the compressor and electronics. Unlike some minor faults that clear themselves, an A0 code usually points to a failed component — most commonly the reactor (DC choke) or the main PCB — that requires a qualified engineer to diagnose and replace. Leaving it unresolved and repeatedly resetting it risks further damage.
How much does it cost to fix a Grant A0 fault code in the UK?
Most homeowners with an A0 fault pay somewhere between £200 and £600 all in, depending on which component is at fault. A reactor replacement tends to sit in the lower-to-mid part of that range, while a main PCB replacement typically costs more — often £300–£600 for parts and labour combined. The engineer's diagnostic visit alone can be £80–£150 before any repair work. In rare cases where multiple components are damaged, costs can exceed £600, but this is uncommon.
Why does my Grant Aerona³ show A0 after a power cut?
A sudden loss of mains power or a power surge can cause a transient voltage spike or drop on the inverter's DC bus, which the controller logs as an A0 fault. In this scenario, simply switching the unit off at the isolator for 60 seconds and restoring power will often clear the code. If the unit runs normally afterwards and the code does not return, no further action is needed. If it reappears, there may be an underlying component issue or your mains supply may be experiencing instability, and an engineer should investigate.