A B737 CREW; LNDG AT CYYZ; RECEIVED A RWY CHANGE WHICH CAUSED THEM TO REPROGRAM THEIR FMS. EVERYTHING WAS CHANGED BUT THE COURSE; CAUSING THEM TO FLY THROUGH THE LOC.

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I WAS OPERATING A SCHEDULED FLT BTWN EWR AND CYYZ. THE FLT PROCEEDED NORMALLY UNTIL THE ACFT FAILED TO INTERCEPT THE RWY 6R LOC IN CYYZ. PRIOR TO DSCNT; I CONFIGURED THE ACFT RADIOS AND MCP COURSE SELECTOR FOR AN APCH TO RWY 6R. ATIS WAS ADVERTISING A LOC APCH RWY 6R; ILS TO RWY 5. I FLEW THE PUBLISHED ARR TO RWYS 5 AND 6R. THE ARR WAS FLOWN WITH THE AUTOPLT ON AND VNAV AND LNAV ENGAGED. UPON REACHING THE YOUTH INTXN; I ANTICIPATED RADAR VECTORS TO FINAL SINCE THAT IS WHERE THE LNAV COURSE TERMINATED WITH A RTE DISCONTINUITY. HOWEVER; APCHING YOUTH INTXN WE HAD NOT RECEIVED VECTORS; SO I RETUNED THE #1 NAV RADIO TO 112.15 YYZ AND MOVED THE COURSE SELECTOR TOWARD THE 340 DEG COURSE TO FLY INBOUND TO CYYZ. AT THIS SAME MOMENT; THE CTLR GAVE US 'PRESENT HDG AND RADAR VECTORS TO RWY 5.' SO I FLIPPED TO RWY 5 APCH PLATE AND RETURNED THE RADIO; BUT FORGOT THE COURSE. AT SOME POINT THEREAFTER; THE WAY WAS AGAIN CHANGED TO RWY 6R. AGAIN I RETUNED THE RADIO TO RWY 6R 109.1. THINKING THAT THE COURSES WERE THE SAME; I NEVER CHKED IT. ALSO; THE APCH CHKLIST WAS ACCOMPLISHED. A FAIRLY NEW CHANGE TO THE CHKLIST; WHICH DROPPED CHKING THE COURSE SELECTOR; MAY HAVE CONTRIBUTED TO THE PNF FROM MISSING THE ERROR. AS WE WERE TURNING TO INTERCEPT THE RWY 6R LOC; AND DSNDING OUT OF 4000 FT TO 3000 FT; THE CTLR CALLED TFC AS A PRIMARY TARGET (NO TYPE OR ALT INFO) AT OUR 2 O'CLOCK POS. THE FO WAS LOOKING FOR THIS TFC AS WE PASSED THROUGH THE LOC. WITH THE COURSE SELECTOR MIS SET AS WE APCHED THE LOC COURSE WITH VOR LOC ARMED AND THE AUTOPLT ENGAGED; THE ACFT TURNED L AWAY FROM THE INBOUND COURSE AND BEGAN TO PASS THROUGH IT. I SAW THIS HAPPEN AND SAW ON THE MAP DISPLAY THAT WE WERE GOING THROUGH. I DISCONNECTED THE AUTOPLT AND TURNED R TO INTERCEPT THE COURSE. AT THIS SAME MOMENT WE RECEIVED A TA FROM THE TCASII AND THE CTLR CALLED SAYING THAT WE WENT THROUGH AND THAT WE HAD TFC. AS I TURNED BACK TO THE R; THE CTLR ISSUED A CLB TO 4000 FT AND VECTORED US BACK AROUND FOR A SECOND APCH. WE NEVER RECEIVED AN RA FROM TCASII. THE SECOND APCH WAS FLOWN MANUALLY AND VISUALLY. THE MISSED COURSE SELECTOR WAS DISCOVERED AT THE GATE. ANTICIPATING THE RADAR VECTOR AND NOT BEING PREPARED TO FLY DIRECT TO YYZ WAS WHERE THE PROB BEGAN. CHANGING RWYS NEVER HELPS; ESPECIALLY WHEN NOT ONLY ARE THE RADIOS CHANGED; BUT ALSO THE FMC NEEDS TO BE REPROGRAMMED DURING A VERY BUSY TIME IN ANY CASE. LOW LIGHT IN THE COCKPIT MAY HAVE CONTRIBUTED TO THE MISSED COURSE SELECTION.