s5-135U fault

Hi!
i have some problem in my distributed s5-siemens system .
this system consist of s5-plc 928 and 2 EU 183 .
on the 928 CPU i got a stop red light falshing speed (2 a second). and BASP is lighting also . on the interface module card (301-3ab13) in control center which connected to distributed EU red led is lighting .
now i am sure which is cousing the problem .is the CPU or the remote extention unit.
i am not able to go on line with the PG also.
Can somebody guide me through the trouble shoot process
best tziki

In this manual on page 152 a description of the LED on the CPU
http://support.automation.siemens.com/WW/view/en/1085940

The 301-3ab13 will go into stop when the CPU goes to stop.

You have to go on-line and read out the BSTACK and ISTACK to see the fault.