BoilerFaultCodesTechnical description
Difference between flow and return temperature too high: check pump and flow rate.
What causes this fault?
This fault occurs when the water inside the boiler gets too hot because it isn't moving away into your radiators quickly enough. The most common reasons are a failing pump that can't push the water, or a buildup of sludge and scale that has physically blocked the internal pipes. When the temperature difference between the water going out and coming back becomes too large, the boiler shuts down for safety.
This fault is frequently seen at the start of autumn when heating systems are switched back on after being idle all summer, which can cause internal pump components 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.