siemens s7 414-4h redundent CPU problem

dear all experts

one day i found that the master cpu's all leds is flashing and control transfered to the slave one

after trouble shooting the case it came back to service then again hapeened

now i am planning to change that cpu, can you guide me about clear steps and precation to be tanken to install a new one and to make it hot redundent with the existing one.?

thanks

dear experts

if all leds of the CPU 414-4H is flashing and this is as per manual means the CPU is faulty

i have to change it or there is any hope that it can wotk by some how ?
please advise