THE CAPT OF AN A319 ON THE KEPEC 1 ARR INTO LAS RPTS RECEIVING A CONFUSING CLRNC. WHILE ATTEMPTING TO CLARIFY WITH APCH; FO BEGAN DSCNT AND HDG CHANGE ON HIS OWN. APCH CTL QUERIED THEM; THEN ISSUED NEW HDG AND ALT ASSIGNMENT.

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FLT CLRED TO DSND ON THE KEPEC ARR. WITH CLRNC TO CROSS AND MAINTAIN 10000 FT AT IPUMY INTXN AND COMPLY WITH ALL OTHER RESTRS. IPUMY INTXN IS ON THE KEPEC ARR WITH PUBLISHED XING RESTR OF 11000 FT. CAPT WAS NOT CLR IF THE CLRNC TO MAINTAIN 10000 FT IPUMY AND CONTINUE FURTHER CLRNC TO A LOWER ALT ON THE ARRIVAL; OR SIMPLY MAINTAIN 10000 FT UNTIL DIRECTED TO DO OTHERWISE. THE CAPT HAD RECEIVED OTHER CLRNCS IN MANY ARRS AT LAS AND HAD TO STOP DSCNT AND MAINTAIN SPECIAL ALTS AND THOUGHT THIS WAS A SIMILAR SCENARIO. AS FLT NEARED IPUMY INTXN; THE CAPT SAID HE WANTED CLARIFICATION OF THE CLRNC. THE FO (PF) DISAGREED WITH THE CAPT AND THOUGHT THEY SHOULD CONTINUE DSCNT AFTER IPUMY TO CROSS NIPZO AT 9000 FT; THEN KEPEC AT 8000 FT. THE CAPT SAID HE WOULD CALL LAS APCH TO VERIFY CLRNC. COMPLYING WITH THE ORIGINAL CLRNC AND XING IPUMY AT 10000 FT THE CAPT TRIED TO CALL APCH WHILE LOOKING OUTSIDE FOR TFC; APCH WAS BUSY TALKING TO OTHER ACFT. CAPT SCANNED BACK INSIDE AND NOTICED ACFT WAS DRIFTING OFF COURSE TO THE L AND AT 8400 FT INSTEAD OF 9000 FT (PUBLISHED ALT AT NIPZO). CAPT IMMEDIATELY REACHED UP TO TURN HDG KNOB TO THE R TO 090 DEG HDG; STATING 'YOU'RE L OF COURSE; TURNING YOUR HDG BACK TOWARDS NIPZO; HEY IT'S NOT RESPONDING; HEY THE AUTOPLT'S OFF.' THEN LAS APCH CALLED 'FLT XXX; WHERE ARE YOU GOING?' CAPT SAID 'BACK TO NIPZO!' THEN LAS APCH INSTRUCTED US TO TURN TO A 090 HDG AND DSND TO 6500 FT. WHEN ASKED WHY HE TURNED OFF THE AUTOPLT; THE FO RESPONDED SOMETHING ABOUT PAX COMFORT. THE CAPT WAS NEVER TOLD THE AUTOPLT WAS DISCONNECTED NOR DID HE REALIZE IT WAS OFF UNTIL HIS SCAN INSIDE THE ACFT. THE CAPT NEVER TOLD THE FO TO DSND NOR DID HE WANT TO BELOW 10000 FT UNTIL HE HAD CLARIFICATION. CLARIFICATION WAS NEVER GIVEN OR RECEIVED AND THE FO DID WHAT HE THOUGHT WAS RIGHT WITHOUT THE CAPT'S AGREEMENT. OBVIOUSLY THE 2 PLTS WERE NOT IN AGREEMENT. AFTER THE NEW HDG AND ALT WAS ISSUED; A NORMAL APCH AND LNDG TO RWY 25L WAS ACCOMPLISHED WITH NO COMPLICATIONS.