BoilerFaultCodesYour heat pump has stopped working because a sensor that monitors the temperature of the cooling liquid returning to the main unit has developed a fault.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Compressor Refrigerant Return Thermistor (Th) Error (E9)
What causes this fault?
This fault occurs when the system's brain loses track of the temperature of the refrigerant liquid as it flows back into the compressor. The most common reason is a failed electrical sensor that has either burnt out or is sending back scrambled readings that don't make sense. It can also be caused by a loose connection or a damaged wire between that sensor and the main control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like E9 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
Affiliate links — we may earn a commission at no extra cost to you.