What was found
AFTER SELECTION OF WING A-ICE TO "ON" L VALVE LIGHT ILLUMINATED WITH ASSOCIATED MASTER CAUTION. GROUNDED - YES. REF ASA FTD 30-22001 AND ASA FTD 30-18001. THIS FAULT IS A KNOWN ANOMALY ESPECIALLY ON L/H WTAI VALVE DUE TO THE VULNERABILITY OF THE SENSE LINE FREEZING DUE TO BEING ON BOTTOM OF VALVE. OPERATED L AND R WTAI OPS CHECK GOOD. LEFT WTAI OPERATES SMOOTHLY OPEN AND CLOSED. NO 30 DAY HISTORY REF B737-9 AMM 30-11-11.