BoilerFaultCodesTechnical description
NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT
What causes this fault?
This fault occurs when the boiler's internal computer detects that the 'return' pipe is hotter than the 'flow' pipe, which is the opposite of how a heating system should function. The most common reason is that one of the temperature sensors has become faulty or unclipped, but it can also be triggered by a pump failure or a significant blockage that is pushing hot water back the wrong way.
This fault often appears when the heating is first turned on after summer, as pump components can seize during periods of inactivity.
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.
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