Grant E4 Fault Code: Causes, Fixes & Repair Costs
What does the Grant E4 fault code mean?
E4 on a Grant Aerona³ heat pump reports an outgoing (flow) water temperature sensor error: the thermistor measuring the temperature of heated water leaving the heat pump for your radiators or underfloor heating has failed or reads implausibly. This is the reading the whole system steers by — flow temperature is what the controller modulates the compressor against, what weather compensation adjusts, and what protects the plate heat exchanger — so the unit stops rather than heat blind. Grant's documented remedy is the standard one: if the sensor is faulty, it should be replaced, with the diagnosis made by resistance check. Its sibling E5 covers the return water sensor.
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 flow water thermistor Common
The sensor fails open, short or drifted with age — the documented cause, confirmed against the resistance table in minutes.
- Wiring or connector fault Sometimes
Damaged leads or a corroded connector reproduce sensor failure; the loom is checked before parts are fitted.
- Sensor unseated from its pocket Sometimes
A probe dislodged from its pocket reads ambient rather than water temperature — plausible values, wrong behaviour, particularly visible at start-up.
How to fix it
- Power-cycle once at the external isolator DIY safe
Switch the heat pump off at its external rotary isolator, wait about three minutes for the inverter capacitors to discharge, and switch back on. One controlled restart is legitimate; if the code returns, leave it for an engineer.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will measure the flow sensor's resistance against the table, inspect wiring and seating, and replace the faulty sensor — then confirm flow temperature tracks correctly and weather compensation behaves through a heating cycle. 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
- Flow 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–£280, depending on the underlying cause.
Frequently asked questions
What does E4 mean on my Grant Aerona³?
The sensor measuring the heated water leaving the unit has failed — the single reading the heat pump steers everything by, so it stops rather than run blind. An engineer confirms it with a resistance check and replaces it; sensors are among the cheapest heat pump parts.
What's the difference between E4 and E5?
E4 is the outgoing (flow) water sensor, E5 the return water sensor — the pair measures the temperature drop across your heating circuit. Same fault class, same diagnosis, same sort of bill; the code just tells the engineer which probe.
Will I lose hot water too?
While the heat pump is stopped, yes — cylinder reheating pauses along with the heating, unless your system has an immersion heater the controls can fall back on. Worth mentioning when booking if you have no backup.