A FAULTY TCASII RA; IF FOLLOWED; WOULD HAVE CAUSED AN LTT TO DSND TOWARD A COLLISION RATHER THAN MAINTAIN ALT FOR PROPER SEPARATION.

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WHILE IN LEVEL FLT AT 16000 FT; ZLA ADVISED US OF SMT TYPE TFC 12 O'CLOCK AND 10 MI OPERATING VFR AT 15500 FT. WE IMMEDIATELY ESTABLISHED VISUAL CONTACT WITH THE TFC BUT OUR ONBOARD TCASII INDICATED HIS ALT AS 16400 FT RATHER THAN 15500 FT. DUE TO ONLY 500 FT SEPARATION AND 10 MI RANGE; WE WERE UNSURE OF THE TFC'S ACTUAL ALT AND WHETHER IT WOULD PASS US ABOVE OR BELOW. WE THEN ASKED CTR TO VERIFY THE TFC'S ALT AND INFORMED CTR OF OUR TCASII RECEIVED ALT OF THE TFC. DUE TO FREQ CONGESTION; A REPLY WAS DELAYED FROM THE SMT. WE IMMEDIATELY RECEIVED A 'DSND' TCASII RA BUT FORTUNATELY WERE ABLE TO VISUALLY CONFIRM THAT THE TFC WOULD PASS BELOW US. UNBEKNOWNST TO US; THE SMT'S XPONDER WAS INTERMITTENTLY XMITTING FALSE ALT DATA DURING MOST OF HIS FLT. IT WAS FORTUNATE THAT THE SMT WAS IN RADIO CONTACT WITH CTR AND THE INFLT VISIBILITY WAS HIGH OTHERWISE THERE WOULD HAVE BEEN A COLLISION WHILE COMPLYING WITH OUR ERRONEOUS TCASII ADVISORY. TO PREVENT SIMILAR INCIDENTS; CTR AND APCH CTLRS SHOULD IMMEDIATELY HAVE SUSPECT XPONDER ACFT TO 'STOP ALT SQUAWK' AND NOT BE ALLOWED TO REACTIVATE IT FOR TROUBLE-SHOOTING PURPOSES WITH TCASII EQUIPPED ACFT OPS IN THE SAME AREA.