p0597 thermostat

After using Prestone Radiator Flush and Cleaner bottle in 2008 328i, the code p0597 came on. The car has 140K miles. The radiator fan used to come on loud for the past a few days when starting the car and stays on. I checked the expanding tank of coolant and when I was bleeding the system I noticed that the return water pressure was very weak. After flushing the system the pressure became strong and thus the code.
Is there something I have done while bleeding or was it coincidental for the thermostat to fail?