Siemens s5-115 fault

We have a Siemens S5-115 944 which a couple of times a week faults out. All 4 red L.E.Ds on the processor are lit. The LEDs are identified as ST, QV, ZY and BA.
Reseting the CPU clears the fault and the machine runs ok again.
Any ideas what the LEDs mean or what could cause this?

115U CPU943/944

The LEDs are:

ST = stop mode
QV = I/O timeout
ZY = scan time exceeded
BA = outputs disabled

Is the stop LED steady, flashing or flickering?

Have you output the ISTACK and analysed it?

The stop LED is steady.

Not sure what you mean by "Have you output the ISTACK and analysed it?"

Problem is I'm not familiar with Siemens. I'm pretty new to this company. Lot's of experience with AB and Mitsubishi, but not S5.

Hello potter;
You should start by opening the Siemens support website (www4.ad.siemens.de), go to Product Support, search for and download the CPU 944 manual:
6ES5 998-0UF23.pdf

Chapter 5 is all about diagnostics, using the LEDs and the interrupt stack (I-Stack). The I-stack is available through the Step5 programming software, in the online functions. An explanation of the cause of faults is also included in the manual, so you can see the origin of your problem.
This is an excerpt:
Hope this helps,
Daniel Chartier