AN ACR FLC FLYING CRJ AT IAD FAILED TO INTERCEPT THE LOC COURSE AS INSTRUCTED BY ATC AND WAS GIVEN ADDITIONAL FLT ASSIST TO THE FINAL APCH COURSE.

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WE WERE CLRED FOR THE ILS 1L AT IAD. THE ARPT WAS VMC WITH A SCATTERED TO BROKEN LAYER BTWN US AND THE RWY. WHEN CLRED FOR THE APCH; I (THE PF) PROPERLY ARMED THE NAV EQUIP ON THE FCP; WHILE THE EFIS SHOWED THE LOC AND GS CAPTURED. WE HAD A FULL R DEFLECTION OF THE LOC NEEDLE. I REALIZED THAT THE SIT WAS NOT NORMAL. (IT SHOULD NOT HAVE SHOWN A FULL DEFLECTION WITH A LOC 2 CAPTURED DISPLAYED.) ALMOST SIMULTANEOUSLY; THE CAPT; THE CTLR; AND I REALIZED THAT WE HAD FLOWN THROUGH THE INBOUND COURSE. THE NEEDLE MADE A RAPID SWING TO A FULL L DEFLECTION. I TURNED OFF THE AUTOPLT AND MADE A L TURN AT ABOUT THE SAME TIME THE CTLR TOLD US TO MAKE A L TURN QUICKLY. THE RWY WAS NOW IN SIGHT AND WE LANDED WITHOUT FURTHER INCIDENT. I CAN ONLY SPECULATE AS TO WHAT MAY HAVE CAUSED THIS SIT. ON THE CARJ WE HAVE THE ABILITY TO HOLD DME INFO FROM A VOR. WHILE THIS IS AN ILS DME APCH; I NOTICED (AFTER ARMING THE FCP) THAT I WAS NOT RECEIVING DME INFO. I THEN SWITCHED BRIEFLY TO THE VOR AND PRESSED THE HOLD BUTTON ON THE NAV RADIO AND THEN BACK TO THE ILS FREQ. MY FIRST THOUGHT WAS THAT I HAD NOT SWITCHED BACK TO THE ILS FREQ; BUT THE CAPT AND I BOTH VERIFIED THAT THE PROPER FREQ WAS SELECTED. PERHAPS (AND ONCE AGAIN I SPECULATE) WHEN I SWITCHED TO HOLD THE VOR DME; THE COMPUTER LOCKED ONTO THE VOR SIGNAL AND RETAINED IT EVEN AFTER I SWITCHED BACK TO THE ILS FREQ; THE ENTIRE TIME SHOWING LOC 2 CAPTURED. I WILL BE IN A DISCUSSION WITH COMPANY PERSONNEL ABOUT THE SIT.