A B757 FLT CREW EXPERIENCES NAV INST DISCREPANCY DURING AN APCH.

Narrative

A VMC DAY WITH HAZY CONDITIONS. WE WERE CLRED TO INTERCEPT LOC RWY 4R. FO WAS AT THE CTLS WITH AUTOPLT ENGAGED. ACFT CAPTURED LOC; BUT FLEW THROUGH THE COURSE AND TURNED BACK TO THE N TO GET ON COURSE. DURING THIS TIME THE CDI WAS INDICATING NORMAL (IE; SHOWING US TO THE R OFF COURSE WITH CORRECTION TO CTR). THE FO HAD LNAV SELECTED ON HIS HSI AND I HAD LOC SELECTED ON MY HSI DISPLAY. LOC COURSE IS 036 DEGS WHEN I LOOKED BACK AT THE HSI A SHORT TIME LATER; THE ACFT WAS STILL MAINTAINING THE HEADING TO INTERCEPT (I THINK ABOUT 015 DEGS OR SO); BUT MY LOC CDI WAS TO THE R ABOUT 1 DOT OFF CTR. I QUESTIONED OUT LOUD AS TO WHAT 'THIS THING IS DOING.' THE FO CHKED HIS HSI LNAV DISPLAY AND IT TOO INDICATED WE WERE L OFF TRACK. HEADING MODE WAS SELECTED AND A TURN WAS MADE TO THE R (ABOUT 070 DEGS) IN AN EFFORT TO CORRECT TO COURSE. THE CTLR THEN ASKED IF WE WERE ESTABLISHED AND I TOLD HIM WE WERE ABOUT 1 DOT OFF. AFTER A MIN OR SO WE WERE STILL INDICATING L OFF COURSE ON BOTH LOC AND LNAV; WITH THE LOC NOT MOVING TOWARD CTR. AGAIN WE XCHKED LNAV AND LOC AND BOTH INDICATED L OFF COURSE. AROUND THIS TIME THE CTLR SAID HE SHOWED US 8 NM R OFF COURSE AND HAD US BREAK OFF APCH AND REENTER. (WE WERE NOW ABOUT 15 NM FROM BOS.) THE SECOND APCH WAS FLOWN NORMALLY.