BE58 PLT INADVERTENTLY SHOWN ON RADAR AS IN CLASS B AIRSPACE WHEN HIS MODE C XPONDER XMITTED AN ALT NOT CORRESPONDING WITH THE ALT SHOWN ON HIS ALTIMETER.

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I DEPARTED BOULDER MUNICIPAL ARPT (1V5) RWY 8 AND CLBED TO 7400 FT MSL TOWARD JEFFCO ARPT ENRTE TO CENTENNIAL ARPT ON A PART 91 FLT (1 PLT ONLY; NO PAX). BECAUSE JEFFCO TWR WAS CLOSED; I MADE A RADIO CALL ON 118.6 AND GAVE MY POS AND INTENTIONS TO FLY OVER THE ARPT AT 7400 FT MSL. AGAIN I RPTED AT 7400 FT OVER THE ARPT AND AGAIN WHEN I WAS 6 MI S OF THE ARPT. AFTER MY LAST RPT OF RPTING AT 7400 FT; I IMMEDIATELY TUNED IN CENTENNIAL ATIS WHILE I WAS FLYING A COURSE W OF THE DENVER CITY PROPER AND W OF I-25. AT 10 MI NW OF CENTENNIAL ARPT AND THEN FLEW DIRECTLY TOWARD CHERRY CREEK RESERVOIR. DURING THAT TIME I WAS LISTENING TO CENTENNIAL ATIS; I NOTICED AN ACFT WITH ITS LNDG LIGHT ON AT 12 O'CLOCK POS COMING STRAIGHT TOWARD ME AND I DECIDED TO CLB TO 7700 FT MSL. AT 15 MI ON MY GPS; I CALLED CENTENNIAL TWR 118.9 AND RPTED MY POS AND INTENTIONS. THE TWR TOLD ME AT THAT TIME THAT DEN DEP HAD CALLED HER AND SAID I HAD BUSTED A CORNER OF THE CLASS B AIRSPACE AT OVER 8000 FT MSL. I CAME BACK TO HER AND SAID THAT; HOW COULD THAT BE TRUE SINCE MY HIGHEST ALT I HAD BEEN WAS 7700 FT MSL. SHE CAME BACK TO ME AND ASKED ME WHAT MY ALT WAS AND I SAID 7700 FT. SHE THEN ASKED ME TO RPT OVER CHERRY CREEK RESERVOIR. AT THAT POINT I WAS CLRED TO LAND ON RWY 17R. THE TWR NEVER MENTIONED THE ATC INCIDENT WITH ME AGAIN; NOR DID SHE ASK ME TO CALL ZDV. I BELIEVE THE CAUSE OF THE PROB MIGHT HAVE BEEN AN ERROR IN THE MODE C XPONDER IN MY AIRPLANE SINCE THE BEECHCRAFT BARON HAD BEEN OUTSIDE ALL NIGHT AND WAS COLD SOAKED OR THE ZDV CTR HAD A PROB WITH THEIR RECEIVING MY 1200 VFR XPONDER CODE. TO PREVENT THIS RECURRENCE AGAIN; I WILL TALK TO DEN DEP AND COMMUNICATE WITH THEM THROUGH THIS CORRIDOR.