What was found
DECLARED EMERGENCY DUE TO FUMES IN THE COCKPIT PASSING THROUGH FL190 DURING THE APPROACH. DONNED OXYGEN MASKS AND CONTINUED TO DFW. APU WAS PREVIOUSLY DEFERRED FOR LOAD COMPRESSOR CONTAMINATION. DFW MAINTENANCE FOUND APU CONTROL C/B PUSHED IN, ALLOWING APU TO RUN. SUSPECT APU WAS RUN DURING AIRCRAFT MOVE TO THE GATE PRIOR TO DEPARTURE FROM ATL. PERFORMED NO-GO ECS DECONTAMINATION. APU DEACTIVATED AGAIN. APU LATER REPLACED. (LP 8532928, 8532926 - AIRCRAFT WAS GROUNDED).