output module plc analog input and relay output modules problems

Upper and lower limits of analog input values ​​are set to 20 and 0 , had  plc analog io module everything normal, in the absence of the analog signal access , the software displays a value of 0 , there is signal access , they also have a normal display, but under normal use two or three days , the upper and lower limits remain unchanged, the analog input value in the absence of wiring is 6.8 , all the channels I have tried are 6.8 , then the company gave me a new simulation input module , installed after the beginning is normal, but it is the same problem in a couple of days later , I ask an expert what causes ? Urgent
Also this PLC as well as a relay output module , 2, 3 and 4-channel connection of solenoid valves, found often there will be a problem , sometimes when I start solenoid valve, status lights all of a sudden relay plc cpu modules  output modules are extinguished , the network does not pass if the output has an inductive load , inductive load at the moment there pass off thousands fleeing negative voltage power module and power module fleeing after each power unit , causing damage to electrical components . So best to separate inductive load power . I had always felt that the problem is not , then burned two PLC know, sometimes it is really necessary to separate power supply.