A PVT ACFT'S MODE C MALFUNCTIONED IN CLASS B AIRSPACE. AFTER POSSIBLY CAUSING A TCASII ALERT; AND GREAT CONCERN ON THE ARPT OF THE CTLR; THE MODE C USED WAS DISCONTINUED.

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AT ABOUT XA45 ON XX/SAT/95 I CALLED FOR TAXI WITH FALCON GND WITH CODE (ATIS). THE ALTIMETER SETTING WAS 30.00 INCHES. I ALSO; ASKED FALCON GND FOR A XPONDER CODE AND APCH FREQ TO TRANSITION PHOENIX CLASS B AIRSPACE TO THE NW; DEST; LANCASTER; CA. FALCON GND INFORMED ME THAT THEY WERE UNABLE; HOWEVER; THEY WOULD EXPEDITE A FREQ CHANGE AND ADVISE ME OF THE APPROPRIATE APCH FREQ. AFTER TKOF; I TURNED NW AND; AFTER A SHORT DELAY; I CALLED FALCON TWR FOR A FREQ CHANGE WHICH WAS IMMEDIATELY AUTHORIZED. AFTER MY CALL TO PHOENIX APCH I WAS GIVEN A CODE AND ALTIMETER SETTING OF 29.91 INCHES AND AFTER THEY ESTABLISHED RADAR CONTACT; THEY CLRED ME TO 'AT OR BELOW 5500 FT.' PHX APCH ASKED FOR ME TO 'SAY ALT' AND I RESPONDED '4900 FT FOR 5500 FT.' PHX APCH APPEARED TO RESPOND WITH PANIC -- THAT I WAS 1000 FT HIGHER THAN I STATED; AND PROCEEDED TO VECTOR ME WITH LARGE HDG CHANGES AND A REDUCTION TO 4000 FT. THE CTLR'S APPARENT PANIC WAS FURTHER EVIDENCED BY THE FACT THAT HE HAD ALREADY ACKNOWLEDGED OUR CORRECT ACFT TYPE AND CALL SIGN MENTOR CALL SIGN AND NOW; WAS CONFUSED; CALLING US BONANZA; AND NOT SURE OF OUR CALL. AFTER SEVERAL HDG CHANGES PHOENIX APCH GAVE ME HDG OF 280 DEGS BUT BEFORE THINGS FINALLY SETTLED DOWN HE GAVE ME A PHONE NUMBER TO CALL; THEN ASKED ME IF I WANTED FLT FOLLOWING. DUE TO THE CONFUSION AND POTENTIAL MODE C ERROR; I DECLINED. HE TRANSFERRED ME TO LUKE APCH AFTER SEVERAL MINS ELAPSED. LUKE CLRED ME TO MY REQUESTED CRUISING ALT OF 6500 FT AND REQUESTED THAT; DUE TO MY APPARENT MODE C PROB; TO TURN XPONDER TO MODE A. THE LUKE CTLR WAS REFRESHINGLY CALM AND THE REST OF THE FLT WAS NORMAL. AFTER ARR AT ROSAMOND; CA L00; I CALLED AS REQUESTED; AND TALKED TO THE WATCH SUPVR. HE SAID THAT THE PROB WAS RELATED TO A POSSIBLE EVASIVE ACTION; THAT MIGHT HAVE BEEN TAKEN BY ANOTHER ACFT RESPONDING TO A TCASII ALARM. I EXPLAINED THAT I WAS UNAWARE OF A MODE C PROB WITH THE ACFT THAT I WAS FLYING; HOWEVER I PLANNED TO HAVE THE ENCODER CHKED OUT. THE NEXT DAY I TOOK THE ACFT TO A REPAIR STATION LOCATED AT CALIFORNIA CITY ARPT L71. IT WAS DETERMINED THAT ALTHOUGH THE PLT STATE CHK WAS IN COMPLIANCE; THE ENCODER HAD DRIFTED OFF 1400 FT. THE ENCODER WAS READJUSTED; AND A COMPLETE PITOTSTAT WAS PERFORMED. IT SHOULD BE NOTED THAT BOTH OF THE ALTIMETERS; WERE IN TOLERANCE. I FEEL THAT THE PHOENIX CTLR SHOULD HAVE TRUSTED MY ALTIMETER READBACK; NOT THE MODE C.