As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI WHILE STARTING ENGINE APU DOES NOT HAVE PNEUMATIC AIR FOR ENGINE START,A/C THEN RETURNED TO THE GATE (BTB) AND MTX GROUNDED A/C. MTX DEFERRED APU AIR FOR DEPARTURE IAW MEL ABX-49-11-1-1. AND RETURNED A/C TO SERVICE. A/C DEPARTED