Your boiler is having trouble reading the temperature of the water returning to it, which prevents it from heating your home correctly.

return NTC error (2nd CH)

What causes this fault?

This fault occurs when the boiler's internal sensors detect an inconsistent temperature reading or a slow rate of water flow through the system. This is often caused by a buildup of debris or lime scale within the heat exchanger, or a sensor that is starting to fail and providing inaccurate data. Much like a safety check, the boiler pauses briefly to ensure it doesn't overheat before attempting to fire up again.