Your boiler's internal pump is failing to stay on long enough to safely dissipate heat after the burner turns off, which can cause the system to overheat.

What causes this fault?

This fault occurs when the boiler's control system detects that the internal pump has stopped spinning immediately after the heat cycle ends, rather than continuing to circulate water to cool the heat exchanger down. The most common reason is a failing pump motor that has developed a dead spot or mechanical resistance, preventing it from performing its crucial post-run cooling phase. In some cases, a communication error from the main circuit board means the pump isn't receiving the command to keep running.