Narrative:

We were at cruise just southwest of ZZZ when the purser called to report an acrid sulfur smell near row 10. No flames or smoke associated with smell. The aircraft had ceiling lights inoperative and deferred at rows 9-12. We suspected a problem with lighting in the area or a ballast unit. Asked the purser to report back in a few mins with an update. In the meantime; we ran the cabin lighting fire/smoke checklist as a precaution. Purser called back to report odor dissipating a short time later. We took a few mins to review other checklists that may become appropriate if indeed we do have a serious problem. (Electrical fire; smoke fumes odor; etc). We had begun the descent to ZZZ1 when the purser called back saying the odor had returned and now it was stronger. At this point we ran the electrical fire checklist; making sure all associated breakers with cabin lighting had been pulled; shut off the recirculation fan; galley power and split the busses with the bus xfer switch. We declared an emergency with ATC who gave us direct ZZZ1; requested fire trucks; notified dispatch; briefed the flight attendants for a cabin advisory that may escalate to an evacuate/evacuation; and briefed the passenger. Communication was excellent at all times with the flight attendants. As we approached ZZZ1 the purser called to report odor dissipated. We reiterated that this would be a cabin advisory unless they noticed smoke or flames and to call immediately upon landing with an update. We landed uneventfully. The purser called stating no smoke or fire so we made a 'remain seated' PA. The fire trucks followed us to the gate and debriefed. We then debriefed the flight attendants after the passenger deplaned. We all felt the situation went very well with excellent communication. In talking to the mechanics; they found the equipment cooling exhaust fan had burned itself out and popped the breaker while we were at the gate. They replaced the fan and showed us the defective fan that had a strong acrid odor and loose bearings. The fan blades had cut into the housing unit. With a new fan installed we proceeded on to ZZZ2 uneventfully.

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Original NASA ASRS Text

Title: A B737-300 IN CRUISE AT FL350 DECLARED AN EMER AND RETURNED TO THE DEP ARPT DUE TO FUMES IN CABIN. CAUSED BY A FAILED EQUIP COOLING FAN.

Narrative: WE WERE AT CRUISE JUST SW OF ZZZ WHEN THE PURSER CALLED TO RPT AN ACRID SULFUR SMELL NEAR ROW 10. NO FLAMES OR SMOKE ASSOCIATED WITH SMELL. THE ACFT HAD CEILING LIGHTS INOP AND DEFERRED AT ROWS 9-12. WE SUSPECTED A PROB WITH LIGHTING IN THE AREA OR A BALLAST UNIT. ASKED THE PURSER TO RPT BACK IN A FEW MINS WITH AN UPDATE. IN THE MEANTIME; WE RAN THE CABIN LIGHTING FIRE/SMOKE CHKLIST AS A PRECAUTION. PURSER CALLED BACK TO RPT ODOR DISSIPATING A SHORT TIME LATER. WE TOOK A FEW MINS TO REVIEW OTHER CHKLISTS THAT MAY BECOME APPROPRIATE IF INDEED WE DO HAVE A SERIOUS PROB. (ELECTRICAL FIRE; SMOKE FUMES ODOR; ETC). WE HAD BEGUN THE DSCNT TO ZZZ1 WHEN THE PURSER CALLED BACK SAYING THE ODOR HAD RETURNED AND NOW IT WAS STRONGER. AT THIS POINT WE RAN THE ELECTRICAL FIRE CHKLIST; MAKING SURE ALL ASSOCIATED BREAKERS WITH CABIN LIGHTING HAD BEEN PULLED; SHUT OFF THE RECIRCULATION FAN; GALLEY PWR AND SPLIT THE BUSSES WITH THE BUS XFER SWITCH. WE DECLARED AN EMER WITH ATC WHO GAVE US DIRECT ZZZ1; REQUESTED FIRE TRUCKS; NOTIFIED DISPATCH; BRIEFED THE FLT ATTENDANTS FOR A CABIN ADVISORY THAT MAY ESCALATE TO AN EVAC; AND BRIEFED THE PAX. COM WAS EXCELLENT AT ALL TIMES WITH THE FLT ATTENDANTS. AS WE APCHED ZZZ1 THE PURSER CALLED TO RPT ODOR DISSIPATED. WE REITERATED THAT THIS WOULD BE A CABIN ADVISORY UNLESS THEY NOTICED SMOKE OR FLAMES AND TO CALL IMMEDIATELY UPON LNDG WITH AN UPDATE. WE LANDED UNEVENTFULLY. THE PURSER CALLED STATING NO SMOKE OR FIRE SO WE MADE A 'REMAIN SEATED' PA. THE FIRE TRUCKS FOLLOWED US TO THE GATE AND DEBRIEFED. WE THEN DEBRIEFED THE FLT ATTENDANTS AFTER THE PAX DEPLANED. WE ALL FELT THE SIT WENT VERY WELL WITH EXCELLENT COM. IN TALKING TO THE MECHS; THEY FOUND THE EQUIP COOLING EXHAUST FAN HAD BURNED ITSELF OUT AND POPPED THE BREAKER WHILE WE WERE AT THE GATE. THEY REPLACED THE FAN AND SHOWED US THE DEFECTIVE FAN THAT HAD A STRONG ACRID ODOR AND LOOSE BEARINGS. THE FAN BLADES HAD CUT INTO THE HOUSING UNIT. WITH A NEW FAN INSTALLED WE PROCEEDED ON TO ZZZ2 UNEVENTFULLY.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.