As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WING ANTI-ICE L-VALVE AND MASTER CAUTION WHEN TURNED ON INFLIGHT. GROUNDED - YES. PERFORMED L/H WING TAI HEALTH CHECK REF B737-9 AMM TASK 30-11-11-200-801. DURING REMOVAL OF INLET TUBE FOUND STANDING WATER. REMOVED WATER BY BLOWING N2 THROUGH LINE. INSPECTED LINE FOR DAMAGE WITH NO DEFECTS NOTED. C/W OPS CHECK WITH NO DEFECTS NOTED. OPS CHECK GOOD. FAULT DID NOT RETURN.