E90 320i coolant light staying on

[maverick328]

Hi all, my dad's 2010 E90 320i had a coolant low light come on recently. We got some BMW coolant but it hardly took any coolant before the stalk popped up to indicate that it's full. I then closed the tank cap and started up but the coolant light is still on. I suspected the sensor so I had it replaced but the light is still on and the coolant level is full. Does anyone know what the fix for this is?

Thanks
M