Model of the fryers would be a big help. But if you have a screen coming up with a failure to heat, You probably have a gas fryer and it registered a failed ignition. All fryers have some drift. Perhaps you may be overloading. Fryers come in many different sizes from 10 Lb countertops to over 100 lb. volume machines. The pound rating is the amount of shortening/fry oil it holds. not the amount of food that can be fried. The more load, the more drop/ drift you will see.
Depending on the type of control and where they put the sensor, there will always be a delay from the set point. Then you also have the lag between call for heat till the oil is at temp.
So the more information you provide us, the more likely we can help you.