B767 CREW HAD ALT OVERSHOOT BECAUSE OF FMC MISPROGRAMmING IN SCT CLASS E AIRSPACE.

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ALTDEV. THE FO WAS FLYING. WE BRIEFED THE CIVET ARR AND AGREED TO USE VNAV FOR THE DSCNT. WE WERE CLRED TO DSND VIA THE CIVET ARR AT AN ASSIGNED SPD OF 250 KTS. I SET 8000 FT IN THE ALT ALERT ANEL WHICH IS THE LAST ALT FOR THE ARR. JUST PRIOR TO CIVET; ATC ASSIGNED A HDG OF 220 DEGS AND AN ALT OF 12000 FT; WHICH I SET ON THE ALT ALERT PANEL. A FEW MINS LATER; ATC TOLD US TO GO DIRECT TO AND CROSS ARNES AT 10000 FT OR ABOVE; CLRED TO DSND VIA THE CIVET ARR; SLOW TO 220 KTS. I SET 8000 FT ON THE PANEL ASSUMING VNAV WOULD AGAIN BE USED. A FEW SECONDS HAD PAST. ATC CLRED US AFTER FUELR TO INTERCEPT THE ILS TO RWY 25R. WE WERE SET UP FOR THE ILS TO RWY 25L. I CHANGED THAT IN THE FMC; SET IN THE CORRECT FREQ IN THE ILS RADIO HEAD; AND THEN WAS IN THE PROCESS OF OBTAINING THE APCH PLATE WHEN I NOTICED THAT WE WERE XING ARNES AT 8000 FT INSTEAD OF 10000 FT. WE HAD BEEN IN FLT LEVEL CHANGE INSTEAD OF VNAV AND THE FO LEVELED AT 8000 FT; WHICH WAS SET IN THE ALT ALERT PANEL. I HAD A LONG PERIOD OF HEADS DOWN IN THE LAST FEW MINS OF THE EVENT. I SHOULD HAVE BEEN MORE ATTENTIVE TO THE ACTIONS OF THE FO AND BACKED HIM UP. HE WAS SOMEWHAT NEW TO THE LAX ENVIRONMENT. SUPPLEMENTAL INFO FROM ACN 510741: I WAS FLYING THE LEG. WE WERE CLRED DIRECT TO CIVET AND CLRED FOR THE CIVET 4 ARR. THE CAPT SET 8000 FT IN THE MCP AND WE DSNDED USING VNAV AND LNAV. I DID NOT SET ANY ALT DATA ON THE HSI AND DID NOT WANT TO DSND USING VNAV; SO I SELECTED FLT LEVEL CHANGE. WE WERE THEN TOLD TO SLOW TO 220 KTS; WHICH I DID USING SPD BRAKES. WE THEN LEVELED AT 8000 FT AND CROSSED ARNES AT 8000 FT INSTEAD OF 10000 FT. MY SITUATIONAL AWARENESS WAS NOT WHERE IT SHOULD HAVE BEEN AND I GOT DISTR AND DID NOT CONFIRM OUR CLRED ALTS WITH THE CAPT.