BoilerFaultCodesYour heat pump has stopped working because water is not moving through the outdoor unit properly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Outdoor Unit, No Water Flow, Error
What causes this fault?
This fault occurs when the system detects that water isn't moving through the outdoor unit at the required speed to safely exchange heat. The most common reasons are a build-up of debris or sludge blocking the internal filter, a failure of the main circulation pump, or trapped air pockets within the pipework. When the water stops moving, the unit shuts down to prevent the refrigerant system from overheating or freezing.
DIY fix possible
Check that your central heating pressure gauge is between 1 and 1.5 barEnsure all radiator valves and isolation valves are fully openCheck the outdoor unit for any obvious obstructions or debris
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 9.1.9 — 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.