BoilerFaultCodesTechnical description
The heating return thermistor is detecting an excessive temperature.
What causes this fault?
This fault occurs when the water traveling back from your radiators is nearly as hot as the water leaving the boiler, which prevents the system from cooling down properly. Typically, this happens because the water isn't moving through the pipes fast enough due to a failing pump or a buildup of sludge creating a blockage. It can also be triggered if the sensor responsible for monitoring these temperatures has become faulty and is sending incorrect data to the control board.
This issue often surfaces during the first cold spell of autumn when the heating system is turned on after being dormant all summer, which can cause internal pump parts to seize.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.