Grant A8 Fault Code: Causes, Fixes & Repair Costs
What does the Grant A8 fault code mean?
The A8 fault code on the Grant Aerona³ heat pump indicates that the outdoor ambient air temperature sensor (commonly referred to as the T8 sensor) is returning a reading that is out of range, open-circuit, or short-circuit. The main PCB cannot obtain a valid temperature signal from this sensor, which it relies on to manage compressor operation, defrost cycles, and weather compensation logic. As a safety measure, the unit will typically shut down until a valid reading is restored.
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 or degraded NTC thermistor (T8 sensor) Common
The T8 outdoor air temperature sensor is a small NTC thermistor that can degrade over time or fail outright, producing a resistance reading that is either infinite (open-circuit) or zero (short-circuit). This is the most common cause of an A8 fault and is usually straightforward for an engineer to diagnose with a multimeter.
- Damaged or loose sensor wiring Common
The cable running from the T8 sensor back to the terminal PCB can suffer physical damage, corrosion at the connectors, or simply work loose from its terminal. Vibration from the outdoor unit, seasonal temperature cycling, and moisture exposure all contribute to wiring deterioration over time.
- Moisture or water ingress into the sensor housing Sometimes
Condensation or water ingress around the sensor head — particularly during cold, damp UK winters — can cause erratic or out-of-range readings that trigger the A8 fault even when the sensor itself is not yet permanently failed.
- Main PCB sensor input fault Rare
If the T8 sensor and its wiring both check out correctly, a fault on the sensor input circuit of the main PCB can produce an identical A8 error. This is a less common cause but should be considered after the sensor and wiring have been ruled out.
How to fix it
- Check for any obvious external issues around the outdoor unit DIY safe
Before calling an engineer, take a quick look at the outdoor unit. Make sure the area around it is clear of leaves, debris, or heavy snow that could affect airflow or physically damage sensor wiring. Do not probe or remove any covers.
- Check the system pressure on the buffer vessel or expansion vessel gauge DIY safe
While an A8 fault is sensor-related rather than pressure-related, it is worth confirming your heating system pressure reads between 1.0 and 2.5 bar when cold. If pressure is low, top it up via the filling loop before calling an engineer, as a low-pressure condition can cause additional faults.
- Power-cycle the heat pump via the external isolator DIY safe
Switch the heat pump off at the external isolator, wait at least 30 seconds, then switch it back on. Some A8 faults caused by a temporary sensor glitch or moisture event will clear themselves on restart. If the A8 code returns promptly, the fault is persistent and an engineer is required. Do not repeatedly reset the unit if the code keeps returning.
- Have an engineer measure the T8 sensor resistance heat pump engineer
A qualified engineer will use a multimeter to check the resistance of the T8 NTC thermistor against the expected value for the current outdoor temperature. A reading of open-circuit or near-zero ohms confirms the sensor has failed and needs replacing.
- Have an engineer inspect the sensor wiring back to the terminal PCB heat pump engineer
The engineer will trace the T8 sensor cable from the sensor head to its terminal on the PCB, checking for signs of corrosion, chafing, pinching, or loose connections. Damaged wiring or corroded terminals will be repaired or replaced as necessary.
- Have the T8 sensor replaced if confirmed faulty heat pump engineer
The T8 outdoor ambient sensor is a plug-in NTC thermistor and a relatively low-cost component. Once replaced, the engineer will verify the new sensor is reading correctly before resetting the fault code.
- Have the main PCB assessed if the sensor and wiring are both intact heat pump engineer
If the T8 sensor and its cabling test correctly, the engineer will investigate the sensor input circuit on the main PCB. A faulty PCB input will require the main PCB to be replaced — this is a more involved repair but follows the same diagnostic sequence.
- Allow the engineer to reset the fault code correctly heat pump engineer
On the Aerona³, some fault codes reset automatically once the underlying fault is resolved; others require a manual reset by a qualified engineer via the remote controller or by powering the unit off at the external isolator. Grant UK advise that homeowners should not attempt to reset fault codes themselves, as doing so before the fault is actually fixed can mask the problem.
- Contact an MCS-certified heat pump engineer if the fault persists heat pump engineer
If the A8 code does not clear after the power-cycle check in Step 3, arrange a visit from an MCS-certified engineer with Grant Aerona³ experience. You can locate a Grant-accredited installer at grantuk.com/find-an-installer. Note that any work on the refrigerant circuit requires an F-Gas registered engineer, though an A8 sensor fault does not typically involve the sealed refrigerant system.
Parts you may need
- T8 Outdoor Ambient NTC Temperature Sensor (Grant Aerona³) · from £30
- Sensor Wiring Loom / Connector Kit · from £20
- Grant Aerona³ Main PCB (Terminal PCB Assembly) · from £380
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–£320, depending on the underlying cause.
Frequently asked questions
Can I reset the A8 fault code myself on a Grant Aerona³?
Grant UK's guidance is that fault codes should not be reset by the homeowner, because some codes reset automatically once the fault is fixed and others need to be manually cleared by a qualified engineer. The one exception is a single power-cycle via the external isolator (switching off, waiting 30 seconds, switching back on) — this is safe to try and will clear a temporary glitch. However, if the A8 code returns after the power-cycle, do not keep resetting it; contact your installer or a Grant-accredited engineer to diagnose and repair the underlying fault.
Will my heat pump keep running with an A8 fault, or will it shut down completely?
In most cases the Aerona³ will lock out and stop operating when an A8 fault is active, because the outdoor air temperature reading is essential to the unit's control logic — it drives defrost decisions and weather compensation calculations. Running without a valid outdoor sensor signal could lead to inefficient operation or component stress, so the PCB shuts the system down as a protective measure. You are unlikely to have any heating or hot water until the fault is resolved.
How much does it typically cost to fix a Grant Aerona³ A8 fault?
For the most common cause — a failed T8 NTC sensor — most UK homeowners pay in the region of £150 to £320 all-in, covering the call-out, the replacement sensor (typically £15–£50 for the part), and the engineer's labour to fit and test it. If the wiring is damaged, add a small amount for a connector or loom repair. In the less common scenario where the main PCB sensor input has failed and the board needs replacing, costs will be considerably higher — typically £500 to £750 or more including parts and labour — but this is not the typical outcome for an A8 fault.
How can I prevent the A8 sensor fault from recurring?
Keeping up with annual servicing by an MCS-certified engineer is the single most effective step — a service visit will include checking sensor resistance values and wiring condition before a fault develops. It is also worth ensuring the outdoor unit has adequate clearance and is not subject to water dripping directly onto the wiring connections, which accelerates corrosion. Keeping your unit within its manufacturer warranty and service schedule also means faults are more likely to be caught early and at lower cost.