Thanks for the reply, I had seen the manual PDF, The machine is 7 - 10 years old so configuration must be o.k, the devices have been replaced as has the module. Apart from replacing all the cables what other "fault in the external wiring or input circuit has been detected" could there be? This fault condition does not occur every time the switches operate.
A point guard I/O safety module sometimes shows the RED LED's on the input after the device has switched. What could be the cause of this? Set up is dual channel, equivalent with 1 second discrepancy. Two solenoid valves operating making 2 independent limit switches when valves closed which are wired back to the module. Module itself has been replaced and so have both limit switches (Festo cylinder reed switches) This does not always go into fault, only sometimes but when it does show RED LEDs' the software shows module as healthy. Disconnecting 24v supply and reconnecting solves the problem (inputs show as amber LED). Any help appreciated.