What was found
#1 FUEL TANK IMBAL 4.38AND #2 FUEL TANK AT 5.59NO IMBALANCE ISSUES PRIOR TO THIS . HAPPENED VERY QUICKLY DESCENDING THRU 11,000 FEET INTO PDX LANDED WITH 4.06 IN #1 AND 5.37IN #2. GROUNDED - YES. ON TANK NO. 1, PERFORMED TANK & COMP. UNITS TEST REF B737-800 AMM 28-41-21-710-801. FOUND F6L TANK UNIT OUT OF LIMITS. REMOVED PANEL 532FB. REMOVED AND REPLACED F6L TANK UNIT. REINSTALLED PANEL 532FB REF AMM 28-11-11 PERFORMED SYS. TEST REF AMM 28-41-00-730-801 OPS CK GOOD. REFUELED NO.1 TANK, NO LEAKS NOTED AT PANEL 532FB. 4 CBS PULLED, COLLARED, REMOVED COLLARS & RESET 4 CBS.