Los Angeles (14-2-4) has won five of its last six games. Houston (8-8-3) has just one win in its last eight matches.
Diomande tied it at 1 in first-half stoppage time on a towering header of Nguyen's cross to the far post. Nguyen drew the defense on a fake shot near the penalty spot and found Diomande open at the corner of the 6-yard box in the 49th.
Romell Quioto opened the scoring in the third minute with an individual effort to create space for a deflected shot past Tyler Miller.