ACR FLC ENCOUNTERS APPARENT MODE C XPONDER ERROR.

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THE CTR WAS ADVISED THAT BOTH ATC XPONDERS WERE GOING TO BE RECYCLED (SELECTED OFF AND ON; AND IN THIS CASE; CIRCUIT BREAKERS FOR ATC #1 AND ATC #2 PULLED AND RESET). THIS PROC DID NOT CORRECT THE ALT RPTING ERROR AS CONFIRMED BY ZOB AND LATER ZAU. THE ALT RPTING MODE WAS SELECTED OFF TO PREVENT ATC CONFLICTS. DISPATCH AND MAINT WERE NOTIFIED. NO ADDITIONAL RECOMMENDATIONS WERE AVAILABLE FROM MAINT AND THE ACFT WAS OPERATED FOR THE REMAINDER OF THE FLT TO MSP WITHOUT ALT RPTING. ATC WAS ADVISED OF CHANGE IN OPERATIONAL STATUS. MODE C XPONDER AND CAPT'S ELECTRIC SERVO ALTIMETER REPLACED TO CORRECT ALT RPTING ERROR. SUPPLEMENTAL INFO FROM ACN 461290: ZOB QUESTIONED OUR ASSIGNED ALT OF FL310. WE CONFIRMED WE WERE AT FL310. IN RESPONSE TO HIS QUESTION; WE ASKED ZOB IF THEY SHOWED SOMETHING DIFFERENT. ZOB ANSWERED YES -- WE WERE INDICATING FL315. WE VERIFIED THAT WE WERE INDEED AT FL310 WITH ALL ALTIMETERS SET TO 29.92. AFTER SEVERAL ATTEMPTS AT TROUBLESHOOTING THE PROB; ZOB STILL HAD US INDICATING FL315. WE WERE THEN GIVEN A BLOCK ALT FL310-FL330. OUR COMPANY DISPATCH AND MAINT WERE INFORMED OF OUR SIT. TO OUR KNOWLEDGE; NO CONFLICT HAD OCCURRED.