Grant P3 Fault Code: Causes, Fixes & Repair Costs
What does the Grant P3 fault code mean?
P3 on a Grant Aerona³ heat pump (the 13kW and 17kW HPID13R32/HPID17R32 models) reports a high pressure switch error: the safety switch monitoring refrigerant pressure on the high side of the circuit has either operated or failed its checks. The high pressure switch is a hard-stop protection — refrigerant pressure climbing out of range means heat is not being shed properly, and continuing would endanger the compressor — so its trips deserve respect rather than resets. Grant's documented remedy for a persisting error is replacement of the switch itself; but because a genuinely operating switch can be reporting a real pressure problem (poor water flow, blocked airflow, refrigerant faults), the engineer verifies the cause before condemning the messenger.
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
- Genuine high-pressure event Common
Poor water flow through the plate heat exchanger, a blocked outdoor coil, or refrigerant-circuit problems push head pressure up until the switch trips — the condition the switch exists to catch.
- High pressure switch failed Sometimes
The switch itself can fail or drift — Grant's documented replacement case once genuine pressure problems are ruled out.
- Wiring or connector fault on the switch circuit Sometimes
A poor connection mimics a tripped switch; checked before parts or refrigerant work.
How to fix it
- Check the obvious flow and airflow basics DIY safe
Confirm system pressure is in the green band and the outdoor unit's inlet and outlet are unobstructed — the two homeowner-visible causes of genuine high-pressure trips.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will establish whether the switch tripped for a real reason — measuring water flow, checking the coil and, with F-Gas qualification, the refrigerant circuit's pressures — and replace the switch only if it has failed, per Grant's documentation. 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
- High pressure switch · from £60
- Circulating pump · from £150
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 £120–£420, depending on the underlying cause.
Frequently asked questions
What does P3 mean on my Grant Aerona³?
On the 13kW and 17kW models, the refrigerant high-pressure safety switch has operated or failed. It may be telling the truth — poor water flow or blocked airflow genuinely raises pressure — so the engineer proves the cause before replacing the switch, which is Grant's remedy when the switch itself is at fault.
Is high refrigerant pressure dangerous?
The switch exists precisely so it never becomes so — it stops the compressor before pressures reach harmful levels. What you should not do is repeatedly reset against it: recurring trips mean a real problem shedding heat, and refrigerant-side investigation needs an F-Gas registered engineer.
What usually turns out to be the cause?
Most often something restricting heat transfer: a sludged filter limiting water flow, or a blocked coil limiting airflow. The switch replacement itself is a modest job — the value of the visit is the diagnosis around it.