Grant L4 Fault Code: Causes, Fixes & Repair Costs
What does the Grant L4 fault code mean?
L4 on a Grant Aerona³ heat pump reports a mix water temperature sensor error: the thermistor measuring temperature at a mixing point — used on installations with a mixed (blended) circuit, most commonly underfloor heating running cooler than the radiator circuit — has failed or reads implausibly. The controller blends flow water down to the mixed circuit's target using that reading; without it, it cannot guarantee the underfloor loops are protected from full-temperature flow, so the affected circuit stops. Grant's documented diagnosis is a resistance check with a tester, and the remedy the standard one: a faulty sensor is replaced. This code auto-resets once the reading returns to normal.
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 mix water thermistor Common
The blending-point sensor fails open, short or drifted — Grant's documented check is its resistance measured with a tester against the table.
- Wiring or connector fault Sometimes
Mixing stations often sit in manifolds and cupboards where leads get disturbed; the wiring is proven before the sensor is condemned.
- Sensor unseated at the manifold Sometimes
A probe slipped from its pocket at the mixing valve reads cupboard air, not blended water — plausible numbers, wrong control.
How to fix it
- Note which circuits are affected DIY safe
L4 concerns the blended circuit — typically underfloor heating. If radiators still heat while the underfloor stays cold, that pattern confirms it and is useful to report when booking.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will measure the mix sensor's resistance with a tester (Grant's documented method), inspect its wiring and seating at the mixing valve or manifold, and replace the faulty sensor — the code auto-resets once a valid reading returns. Work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; your installer or a Grant-accredited (G1) installer is the right call, and Grant UK technical support (01380 736920) can assist with diagnosis.
Parts you may need
- Mix water temperature sensor (thermistor) · from £30
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 £100–£260, depending on the underlying cause.
Frequently asked questions
What does L4 mean on my Grant Aerona³?
The sensor at the mixing point — where flow water is blended down for a cooler circuit like underfloor heating — has failed. That circuit stops so the floor loops cannot be fed full-temperature water. An engineer confirms the sensor with a resistance test and replaces it; the code clears itself once the reading is valid.
Why do my radiators work but the underfloor doesn't?
That is exactly the L4 signature: the fault sits on the blended circuit's sensor, so only that circuit is stopped. It is protective — underfloor pipework and floor coverings are specified for low temperatures.
Do I even have a mix circuit?
If your system is radiators-only on a single temperature, possibly not — some L4 configurations relate to how the installation was set up. Quote your model and system layout to the engineer; a sensor configured but not fitted, or vice versa, produces exactly this kind of code.