What was found
LCH- DURING TAXI, THE APU SHUT DOWN WITH APU FAIL EICAS MESSAGE,FOLLOWED BY SMOKE IN COCKPIT and CABIN. DOWNPOWERED A/C. I TRIED TO RESTART AFTER SEVERAL MINUTES,APU SPOOLED ONLY TO 5% FOR TEN SECONDS, THEN SHUT DOWN AGAIN, AIRCRAFT RETURNED TO GATE. LCH- MAINTENANCE RELEASED THE AIRCRAFT IAW MEL. IAH- MAINTENANCE REMOVED AND REPLACED THE APU,PERFORMED A LEAK CHECK, AND, A OPERATIONAL CHECK, NO DEFECTS NOTED,CLEARED THE MEL, AND, THE AIRCRAFT WAS APPROVED FOR RETURN TO SERVICE.