A B737-400 RPTED A MALFUNCTIONING YAW DAMPER WITH NO ABNORMAL INDICATIONS ON YAW DAMPER INDICATOR. INDICATOR REPLACED; BUT NO HELP. POSITIVE FIX WAS REPLACEMENT OF THE YAW COUPLER.

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ON A PAX 121 REVENUE FLT; WE EXPERIENCED A MALFUNCTIONING YAW DAMPER THAT COULD NOT BE DIAGNOSED INFLT. THE ACFT WENT THROUGH A SERIES OF MINOR; YET NOTICEABLE YAW OSCILLATIONS. THE YAW DAMPER SWITCH AND CORRESPONDING ENGAGED LIGHT SHOWED NO SIGNS; DURING FLT; OF A MALFUNCTION. THE PRE-TKOF CHK OF THE YAW DAMPER INDICATION WAS NORMAL; SO IT APPEARS AS THOUGH THE MALFUNCTIONING YAW DAMPER WAS INOP ONLY INTERMITTENTLY. BOTH THE CIRCUIT BREAKERS AND ON/OFF SWITCH WERE RESET TO NO AVAIL. THE FLT CONTINUED NORMALLY TO ITS FINAL DEST. NO MAINT LOGBOOK ENTRY WAS MADE ON THE FIRST LEG BECAUSE ALL INDICATIONS WERE NORMAL. ON THE LAST LEG; IT WAS DETERMINED UPON TERMINATION OF THE FLT THAT THE YAW DAMPER INDICATOR WAS INOP AND A MAINT LOGBOOK ENTRY WAS MADE. THERE SHOULD BE A DEFINITIVE INDICATOR OR LIGHT TO NOTIFY THE CREW THAT THE YAW DAMPER HAS MALFUNCTIONED. THE FLC THEN CAN TAKE STEPS TO ENSURE A MORE COMFORTABLE RIDE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE YAW DAMPER SYS WAS INTERMITTENT BUT THE FINAL POSITIVE FIX WAS THE REPLACEMENT OF THE YAW COUPLER. THE RPTR SAID THIS WAS ON THE SECOND WRITE-UP WHEN THE YAW COUPLER WAS REPLACED.