As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT DECLARED EMERGENCY ON ACCOUNT OF ELECTRICAL /PLASTIC BURNING SMELL MID CABIN. COULD NOT LOCATE SOURCE. SMELL INCREASED AS WE DESCENDED. MAINTENANCE ACCOMPLISHED CARD 05-96-70 737-NG SMOKE/ODOR FUMES INITIAL TROUBLESHOOTING. FIRE DEPT. THERMISTOR INDICATED SEAT 5EF POWER PORT POWER SUPPLY HOT. ISOLATED TO POWER PORT SYSTEM. REMOVED AND REPLACED SDB AND SPB @ 5EF IAW THALES AMM 44-26-66 REV14.