Siemens 155H PLC Problem


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We have a siemens 155H PLC system.... We have two racks of DO cards and Two of DI's... (DO's are 24VDC and DI's are 110VAC)..... Suddenly one DO rack stopped giving any output.... Upon swapping the Extender Unit's power supply we found that the Power supply was the problem.... got it repaired at the siemens centre.... Replaced it in the system and now we find that there is a QVZ error on the PLC..... The IM modules are working fine.... (upon switching on the PLC, the IM module corresponding to the running PLC shows green LED where as the one corresponding to the stand-by PLC shows amber LED or the Stand-by status.... the switching over is also perfect...).... could anyone guess, why that QVZ (time out error) coming... and what could be the possible way to reset it....?? As I understand each time the PLC goes and polls for the IM modules and if it doesn't find one acknowledging, it might give this error..... Is there a way to request the PLC to stop looking for a particular IM module so that one by one it can be isolated as to which IM module is not responding...??