Grant L2 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L2 fault code mean?
The L2 fault code on the Grant Aerona³ air source heat pump indicates that the control system has detected a problem with the inlet (return) water temperature sensor. This NTC thermistor sits on the water circuit returning to the heat pump and feeds continuous temperature data to the main PCB. When the reading falls outside the expected resistance-temperature curve — or disappears entirely — the heat pump raises the L2 alert and may stop operating to protect the system. The code appears on the Aerona Remote Controller LCD alongside a warning icon and a flashing red LED, and a matching two-digit code is shown on the Terminal PCB LED display 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
- Failed or degraded NTC thermistor Common
The inlet sensor's resistance drifts outside the manufacturer's specified temperature-resistance curve, usually because the thermistor element has aged or failed. This is by far the most frequent root cause of an L2 alarm on the Aerona³ platform. The heat pump cannot trust the reading and raises the fault.
- Loose or corroded wiring connector Sometimes
The sensor connects to the main PCB via a plug-in wiring harness. Vibration over time, or moisture finding its way into the connector, can cause high resistance at the joint or an intermittent open circuit — mimicking a failed sensor without the sensor itself being faulty.
- Moisture or water damage around the sensor Sometimes
Condensation or a minor water leak close to the sensor body or along the cable run can track into the connector and cause erratic readings. This is more likely on older installations or where cable management has allowed pooling.
- Main PCB misreading a good sensor Rare
If the sensor and its wiring both test within specification, the analogue input circuit on the main PCB may itself be at fault, causing it to report an incorrect or absent signal. This is the least common cause but needs to be ruled out once the sensor and wiring have been cleared.
How to fix it
- Check for a tripped MCB or isolator DIY safe
Before assuming an internal fault, go to your consumer unit and confirm the heat pump circuit breaker has not tripped. Then check that the external isolator adjacent to the outdoor unit is switched on. If either has tripped, reset it once and observe whether the L2 code returns.
- Clear the area around the outdoor unit DIY safe
Ensure nothing is obstructing airflow around the outdoor unit — leaves, debris or snow can sometimes cause secondary sensor anomalies. A clear 300 mm clearance on all sides is the minimum. Do not spray water into the unit.
- Check system water pressure DIY safe
Low system pressure can affect water flow and temperature sensor readings. Check the pressure gauge on the indoor unit or buffer vessel — it should typically read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top up via the filling loop following your system's instructions. If you are unsure how to do this, ask your engineer.
- Attempt a controlled power reset DIY safe
Switch the heat pump off at the Aerona Remote Controller, then turn off the external isolator and leave both off for at least two minutes. Restore power and observe whether the L2 code clears. Some sensor faults caused by momentary signal dropout will not return after a reset. If the code reappears within a short time, do not continue resetting — repeated resets without fixing the underlying cause can mask the fault history needed for diagnosis.
- Do not attempt to inspect internal wiring or the sensor yourself heat pump engineer
The inlet sensor and its wiring harness are located inside the water circuit pipework and connected to the main PCB inside the outdoor unit. Accessing these components requires removing panels from the outdoor unit and working around live electrical terminals. This must only be carried out by a qualified engineer. Do not open the unit.
- Have a qualified engineer test the sensor resistance heat pump engineer
An MCS-certified heat pump engineer will use a multimeter to measure the sensor's resistance at a known water temperature and compare the result against Grant's specification table. A reading outside the expected range confirms the sensor has failed and needs replacing. The engineer will also inspect the wiring connector for corrosion or damage.
- Replace the NTC thermistor if it tests out of range heat pump engineer
Replacing the inlet water temperature sensor is a straightforward repair for a trained engineer — it is a relatively low-cost part and fitting it typically takes under an hour. Once replaced, the engineer will verify the new sensor reads correctly across the operating range before clearing the fault code.
- Test or replace the main PCB if the sensor is within specification heat pump engineer
If the sensor and connector both pass inspection but the L2 fault persists, the engineer will assess whether the PCB's sensor input circuit has failed. PCB replacement is a significantly more involved repair. The engineer will confirm costs before proceeding.
- Have the fault code formally reset and the system function-tested heat pump engineer
Some L-series codes on the Aerona³ require a manual reset that should only be performed by your installer or service engineer, not the homeowner. Once the underlying fault is repaired, the engineer will reset the code and run the system through a full heating cycle to confirm normal operation before leaving.
Parts you may need
- Inlet water NTC thermistor sensor (Grant Aerona³ compatible) · from £25
- Sensor wiring harness / connector · from £18
- Grant Aerona³ main PCB (Terminal PCB) · from £320
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–£350, depending on the underlying cause.
Frequently asked questions
Can I reset the L2 fault code myself on a Grant Aerona³?
You can try switching the heat pump off at the remote controller and the external isolator, waiting two minutes, then restoring power. If the code clears and does not come back, a temporary signal glitch may have been the cause. However, Grant advise that some fault codes should only be manually reset by your installer or service engineer, and repeatedly resetting without fixing the underlying fault can make diagnosis harder. If the L2 code returns after one reset, contact a qualified heat pump engineer rather than continuing to reset.
Is the L2 fault dangerous — will my heating and hot water stop?
The L2 fault is a sensor error rather than a safety-critical refrigerant or electrical hazard, so it is not dangerous to your home. However, the heat pump may reduce output or stop running until the fault is resolved, leaving you without heating or hot water. It is worth getting it looked at promptly, especially during colder months, rather than leaving it unattended.
How much does it cost to fix an L2 fault on a Grant Aerona³?
For the most common repair — replacing the inlet water NTC thermistor sensor — most homeowners pay in the region of £150 to £350 all-in, covering the engineer's callout, diagnosis and the part. If the wiring connector is the culprit, costs are typically at the lower end of that range. If the main PCB turns out to be faulty, the cost rises considerably — usually £400 to £800 or more depending on parts and labour — but this is a much less common outcome.
Do I need an F-Gas engineer to fix an L2 sensor fault on my Grant Aerona³?
The inlet water temperature sensor sits on the water-side circuit and does not involve the refrigerant (sealed) circuit, so F-Gas certification is not specifically required for the sensor replacement itself. However, you should still use an MCS-certified heat pump engineer who is trained on the Aerona³ platform. Using an uncertified person risks voiding your manufacturer warranty and may affect your MCS installation records. Grant UK also has a network of accredited G1 service engineers who can be contacted through their website.