Circulating fan runs constant won't shut off. Thermostat shut off. Replaced blower motor circuit board, still runs all the time. What now

Does the system work in the heating mode? Many units with electronic control are designed to run the blower continuously in the event of an open temperature limit device.

No, the system does not work in the heating mode. I did as it turns out have an open limit switch. It tripped because of a defective combustion fan that I now have to replace,

Thank you for your answer.