Grant L3 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L3 fault code mean?
The L3 fault code on the Grant Aerona³ air source heat pump indicates a problem with the outdoor ambient air temperature sensor (commonly referred to as the T3 thermistor). This sensor continuously feeds the heat pump's control board data about the outside air temperature, which the unit relies on to manage heating output, weather compensation curves, and defrost cycle timing. When the control board receives a reading that falls outside its expected range — or receives no signal at all — it logs the L3 fault and may reduce performance or shut the unit down to prevent further damage.
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 outdoor thermistor Common
The outdoor temperature sensor is a small NTC thermistor whose electrical resistance changes predictably with temperature. Over time — particularly in exposed outdoor locations — the component can degrade, produce an out-of-range resistance value, or fail open or short circuit. The control board then flags L3 because the reading no longer makes sense. This is the most frequent root cause of an L3 alarm.
- Damaged, loose or corroded wiring Common
The sensor wiring runs from the outdoor unit back to the main PCB and is constantly exposed to moisture, UV and temperature cycling. Physical damage to the cable sheath, a loose connector at either end, or corrosion on the terminal pins can all interrupt or distort the signal. Intermittent L3 faults that clear and return are a classic sign of a wiring or connector issue rather than a fully failed sensor.
- Moisture or condensation ingress at the connector Sometimes
The outdoor environment means that water can track into plug-and-socket connectors over time. Even small amounts of moisture can bridge connector pins, shift resistance readings and trigger the fault. This is especially common after prolonged wet weather or in installations where drainage around the outdoor unit is poor.
- Main PCB misreading the sensor signal Rare
If the thermistor and its wiring both test within specification, the fault may originate on the control board itself — for example, a faulty analogue input channel or a damaged signal-conditioning component. PCB faults as the sole cause of an L3 code are relatively uncommon but should be considered after sensor and wiring checks have been eliminated.
How to fix it
- Check the Aerona³ remote controller display and note the exact fault code shown DIY safe
The Aerona³ displays fault codes as a letter-and-number combination on the remote controller LCD, accompanied by a warning icon and a flashing red LED on the on/off button. A two-digit version of the code also appears on the terminal PCB inside the outdoor unit. Photograph or write down exactly what is displayed — this helps your engineer diagnose the issue faster.
- Do not attempt to manually reset the fault code yourself DIY safe
Grant advises that some L-series fault codes reset automatically once the underlying issue is resolved, while others require a manual reset by a qualified engineer either via the remote controller menu or the terminal PCB. Repeatedly forcing a reset without addressing the root cause can mask the fault and risk further damage to the heat pump.
- Check that the outdoor unit has power and that no MCBs have tripped DIY safe
Go to your consumer unit (fuse board) and confirm that the circuit breaker supplying the heat pump is in the ON position. Also check the outdoor isolator switch near the unit itself. If a breaker has tripped, reset it once. If it trips again immediately, do not retry — leave it off and call an engineer, as this points to an electrical fault.
- Inspect the area around the outdoor unit for obvious obstructions DIY safe
Clear away any leaves, debris, snow or ice that may have accumulated around the outdoor unit or over the fan grille. While an obstruction alone will not typically cause an L3 sensor fault, a blocked unit can stress other components and obscure access to the sensor connector for an engineer.
- Check and top up heating system pressure if it is below the normal operating range DIY safe
Look at the pressure gauge on your indoor unit or buffer vessel. A correctly pressurised system typically reads between 1.0 and 2.0 bar when cold. If it is below 1.0 bar, top up via the filling loop following your installer's guidance. Note that low system pressure is not a direct cause of L3 but should be logged and reported to your engineer as part of the overall system check.
- Contact an MCS-certified heat pump engineer to diagnose and repair the fault heat pump engineer
A qualified engineer will test the outdoor thermistor resistance using a multimeter and compare the reading against Grant's temperature-resistance reference table. They will also check wiring continuity between the sensor and the main PCB, inspect connectors for corrosion or moisture, and if the sensor and wiring are both healthy, assess the PCB. If the thermistor has failed, replacement is generally straightforward; the engineer will apply a small amount of thermal paste to the new sensor to ensure accurate temperature transfer. If the PCB is at fault, they will advise on repair or replacement options.
Parts you may need
- Outdoor ambient air temperature thermistor (T3 sensor) — Grant Aerona³ compatible · from £35
- Sensor wiring harness / connector repair kit · from £20
- Main control PCB — Grant Aerona³ (if required) · from £420
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–£380, depending on the underlying cause.
Frequently asked questions
Will my Grant Aerona³ heat pump keep running with an L3 fault?
It depends on how severely the sensor reading has deviated. In some cases the heat pump will continue to operate in a reduced or degraded mode while logging the fault; in others it will shut down completely to protect the system. Either way you should have the fault investigated promptly, because running without a reliable outdoor temperature reading affects defrost control and overall efficiency.
Can I clear the L3 fault code myself by switching the heat pump off and back on?
You can switch the unit off at the external isolator and back on again — this is one of the permitted reset methods for some fault codes — but Grant advises that L-series faults should be reset by a qualified engineer rather than by the homeowner. If the fault clears and returns, the underlying problem has not been fixed, and cycling the power repeatedly can mask the issue. Always report a recurring L3 code to your installer or an MCS-certified engineer.
How much does it cost to replace the outdoor temperature sensor on a Grant Aerona³?
For most homeowners with an L3 fault, the total cost — covering a callout/diagnostic visit, the replacement thermistor, and one to two hours of labour — typically falls between £150 and £380. Costs towards the higher end of that range usually reflect regional labour rates (London and the south-east tend to run 10–15% higher) or additional time spent tracing a wiring fault. If the main PCB turns out to be the cause rather than the sensor, that is a more significant repair; a replacement PCB for the Aerona³ typically adds £350–£500 or more on top of labour, so it is worth asking your engineer to confirm the diagnosis before authorising parts.
How can I prevent the L3 fault from recurring?
Annual servicing by an MCS-certified engineer is the most effective preventive measure — Grant recommends it to maintain efficiency, protect the warranty and catch component wear early. During a service, the engineer will inspect sensor connectors, check wiring integrity and clean the outdoor unit, all of which reduce the chance of moisture or corrosion-related sensor faults. Keeping the area around the outdoor unit clear of debris and ensuring surface water cannot pool beneath the unit also helps protect outdoor wiring and connectors.