What was found
FWD CAB TEMP CONTROL VERY IRREGULAR AND ERRATIC. WHEN PLACED AT THE 8-9 OCLOCK POSITION TEMP GOES FROM VERY HOT TO VERY COLD. IN ALL PHASES OF FLIGHT. TRIM AIR ON. ALL OTHER TEMP SELECTORS OPS NORMAL. GROUNDED - YES. FOUND FWD PASSENGER CAB TEMP CONTROL SENSOR GRILL TAPED WITH BLUE MASKING TAPE. VERIFIED NO AIRFLOW TO TEMP SENSOR. REMOVED TAPE FROM GRILL. REPLACED TEMP SENSOR FILTER PRECAUTIONARY REF B737-900 AMM 21-62-06. GROUND VERIFICATION TEST PASSED REF AMM 21-60-00.