As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI, LEFT ENGINE START VALVE FAILED TO OPEN. VALVE LIGHT ON OVERHEAD ILLUMINATED "LEFT ENGINE STARTER" EICAS MESSAGE. A/C RETURNED TO GATE. (BTB) A/C DEFERRED LH ENGINE START VALVE FOR DEPARTURE IAW MEL ABX-80-00-2-1. MTX RETURNED A/C TO SERVICE.