SBR1 CREW HAD AN ERRONEOUS XPONDER ALT READOUT IN ZKC CLASS A.

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ON CLBOUT FROM SUS ENRTE TO LRD WE HAD BEEN GIVEN A HEADING FOR CLB AND CLRED TO CLB TO FL310. HEADING AND ALT WERE AFFIRMED. AS WE APCHED FL310 A CLRNC TO GO DIRECT TO CEDAR CREEK VOR WAS GIVEN. AT THIS TIME WE RECEIVED ANOTHER XMISSION TO 'MAINTAIN FL310.' IT SEEMED REPETITIOUS AND LOOKED UP TO SEE THAT WE WERE AT FL309 AND NOTICED THE XPONDER ON #2. I SWITCHED IT TO #1. NOTHING ELSE WAS MENTIONED AND WE WERE LATER CLRED TO FL350 AND THEN LATER TO FL390. IT WASN'T UNTIL LATER IN THE TRIP THAT ZHU ADVISED US TO CALL ZKC. I IMMEDIATELY RECALLED THE SECOND 'MAINTAIN FL310;' XMISSION. I ASKED CTR WHAT OUR MODE C READ ON THE #2 XPONDER AND THEY ADVISED THAT WE WERE SHOWING FL435 WHEN WE WERE ACTUALLY AT FL430 AS CONFIRMED BY THE SAME CTLR AFTER RETURNING THE XPONDER TO #1. I CALLED ZKC WHEN ARRIVING AT LRD AND THEN ADVISED THAT THE SWITCH SHOWED A LOSS OF SEPARATION BTWN US AND ANOTHER ACFT. HEREIN LIES THE PROB. I FEEL THE ZKC CTLR SHOULD HAVE ASKED US WHAT ALT WE WERE DISPLAYING AND RECONFIRMED THAT WE WERE IN FACT AT FL310. HE NEVER ADVISED US THAT OUR MODE C HAD SHOWN A HIGHER ALT. THE FACT THAT I SAW THE SELECTOR SWITCH ON #2 AND SWITCHED IT TO #1 DID NOT TELL ME I HAD AN INACCURATE XPONDER. I ALWAYS HAVE THE XPONDER ON #1 SINCE I ADVISED THE FO (THE PF) TO LEVEL OFF ON MY ALTIMETER. THE ZKC AREA SUPVR TOLD ME THAT THE PAPERWORK HAD ALREADY BEEN SENT FOR CONTINUED INVESTIGATION TO OUR FSDO. I ACKNOWLEDGED AND HE ALSO SAID THAT FROM MY ACCOUNT OF THE EVENT THAT IT APPEARED TO BE AN EQUIP ERROR. I MEL'ED THE #2 XPONDER PER OUR PROCS AND FLEW BACK TO SUS LATER THAT EVENING. ON 2 OCCASIONS I CHKED THE MODE C WITH ZKC ON RETURN TO CONFIRM THE DISCREPANCY. IF THE CTLR HAD ASKED TO VERIFY OUR ALT WE COULD HAVE ENDED AND ANSWERED ANY QUESTIONS THERE AT THAT TIME. THE AREA SUPVR DID SAY THAT THEIR EQUIP HAD SHOWN FL314 ON ONE 'TICK' AND THE NEXT 'TICK' AT FL308. THIS WAS OBVIOUSLY FROM SELECTING THE #1 XPONDER MODE C. THE NEXT DAY MAINT FOUND THE #2 ALTIMETER TO BE IN ERROR APPROX 300-400 FT AND REPLACED THE UNIT.