[M5 Jed]

Anyone know about why the car won't start if coolant is low? I replaced the thermostat on the B7, filled it with 50/50 mix until max level at the coolant expansion tank. The expansion tank for the intercoolers is still empty. I'm wondering if there's a sensor that is triggered to not let the car start of coolant is dangerously low or if intercooler expansion tank is empty.