Thanks! It’s the pump as I suspected: Replace the circulation pump
There is an issue with the circulation pump. First, ensure that power is getting to the motor. Second, to ensure that the impeller is working, check the pump casing for debris. If power is getting to the motor and the pump is clear of debris, the motor may need to be replaced.
Be aware that the circulation pump may be challenging to replace, although many do-it-yourselfers have been successful with the replacement.
Clever system.