Help needed please, K3, KAT500, KPA 500 interface


I have been running a K3-KPA500-KAT500 lash-up for several years with no problems.  Recently a problem has developed that I can't seem to solve.  When I turn on the K3 a banner message appears in the display saying to "turn on the KAT 500".  The banner can't be cleared from the display.  The KAT 500 is already on when powering up the K3 but it still presents the message to power it on.
1.  I removed both the KAT 500 and the KPA 500 and disconnected the ACC cable from the K3.  The K3 operates correctly with nothing attached to it through the ACC jack.
2.  I then re-introduced the cable being (no tuner or amp)  used on the K3 with no problem.  K3 operates properly. I assume the cable is okay.
3.  I then reintroduced the KAT 500 with no power applied to it.  Just re-installing the tuner caused the banner message to appear saying to " turn on the KAT 500.  Applying power to the tuner and turning it on does not change the message, K3 stays locked on the message until I take the KAT 500 (unpowered) out of line.
4.  I then removed the tuner and put the KPA 500 in line.  The amp is disconnected from all power from the mains.  The message reappears on the K3 to turn on the tuner.  The K3 stays locked on the banner message until the non-powered amp is removed.
5.  I have initialized both the KAT 500 and the K3.  I reloaded firmware in the KAT 500 and the K3 and installed known to be good configuration files.
I am led to believe that the problem is in the K3 I/O ACC board but am at a loss as to what to try next. Has anyone else seen this occur?
Any help from the group would be appreciated.  Todd, WA7U
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