B737-800
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Procedure; Incorrect / Not Installed / Unavailable Part
Removing and replacing No. 2 engine fuel oil cooler;  when reinstalling line that was not removed but loosened and moved out of way; did not inspect gasket or remove and replace gasket;  sure I tightened all lines; leak checked at idle while running did not see any leaks.
B737-8 Technician reported not replacing a gasket on an oil line during an oil cooler replacement.