As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MASTER CAUTION ANTI-ICE ILLUMINATED AND EXTINGUISHED WITHIN 2 SECONDS, UNABLE TO DETERMINE WHICH ENG ANTI-ICE SYSTEM WAS TRIGGERED - TAT 9*C, VISIBLE RAIN. GROUNDED - YES. NO STATUS MESSAGES OR FAULTS LOGGED. WITH CREW ASSISTANCE RAN BOTH ENGINES FOR ANTI-ICE SYSTEM OPERATIONAL TEST. NO FAULTS OR DEFECTS NOTED - REF B737-9 AMM 30-21-00-860-002.