S5 cpu 150 timer card prob

A PLC want to stopmode and required a cold restart
We could't download the software again
after changing the 4 CPU cards we could't download the program and all seems fine
But the timers did'nt run for all subroutines ?
After changing the timercard the problem was fixed!
But we lost al lot of time figuring that out ! How could we have found that problem earlier ? PLC stayed in run during a hardware failure ?

Are you talking about the S5-110U series PLC with the hardware timer card ?
Or is it a S5-155U series ?
Or ... ?

S5 150u

I didnt even know that there was a 150 series before the 155.
Appears that there was also a 130 before the 135.

There are absolutely no manuals to be found on Siemens website about these two systems. 150 was discontinued in 1993.