A B757 ON LNDG GEAR EXTENSION HAD 3 GOOD EXTEND LIGHTS FOLLOWED BY R HALF OF EACH LIGHT OUT. AT 400 FT AGL GOT AURAL 'TOO LOW GEAR' WARNING. EXECUTED AN IMMEDIATE GAR.

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ILS RWY 16R APCH (VISUAL). EXTENDED LNDG GEAR PER COMPANY SOPA/SMAC. RECEIVED 3 GOOD GREEN GEAR EXTENDED LIGHTS FOLLOWED BY THE R HALF OF EACH LIGHT GOING OUT. L SIDE REMAINED ILLUMINATED. NO OTHER EICAM GEAR MESSAGES OR STATUS MESSAGES. AT APPROX 400 FT AGL GOT AN AURAL 'TOO LOW GEAR' WARNING. WE EXECUTED AN IMMEDIATE GAR PER COMPANY PROCS. AFTER GEAR WAS RETRACTED AND UPON LEVELOFF AT 5000 FT WE RECEIVED A LNDG GEAR/MONITOR STATUS MESSAGE. COMPANY DISPATCH AND MAINT CTL WERE CONTACTED AND THE INDICATIONS WERE DISCUSSED. IT WAS AGREED THAT WITH THE AURAL INDICATIONS; A VISUAL CHK BY TWR ON OUR SECOND APCH; THE LACK OF OTHER INDICATIONS OF AN UNSAFE GEAR THAT IT WAS PROBABLY A FAILURE OF ONE SIDE OF THE 2 SENSORS OF THE LNDG GEAR MONITOR SYS. A SUCCESSFUL LNDG GEAR WAS MADE WITH THE GEAR OVERRIDE SWITCH ACTIVATED TO PREVENT A TOO LOW WARNING GEAR AURAL WARNING. UPON TOUCHDOWN THE L REVERSER; AUTOSPOILERS FAILED TO DEPLOY. THE 'AIR GND SENSOR' LIGHT ILLUMINATED. THE PROBE HEAT ECAM MESSAGE ILLUMINATED. AN EMER HAD NOT BEEN DECLARED DUE TO THE BELIEF IT WAS ONLY A FAULTY SENSOR; HOWEVER; I DID HAVE THE ARPT VEHICLES STAND-BY AS A PRECAUTION. I ALSO BRIEFED THE CABIN CREW OF THE NATURE OF THE PROB AND TO BE PREPARED IF SOMETHING UNEXPECTED OCCURRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PROB WAS CAUSED BY THE LNDG GEAR POSITIONING SENSING. THE GEAR EXTENSION WAS OK BUT THE AIRPLANE GEAR POS SENSING AND THE GND SHIFT SENSING WAS FAULTY AS THE R THRUST REVERSER DID NOT DEPLOY AND THE GND SPOILERS HAD TO BE DEPLOYED MANUALLY. THE RPTR STATED MAINT HAS NOT ADVISED THE CREW OF THE EXACT COMPONENT IN THE SENSING CIRCUIT THAT FAILED.