A B737-800 MECHANIC FORGOT TO DISCONNECT #1 ENG NOSE COWL ANTI-ICE VALVE CANON PLUG AFTER LOCKING VALVE IN THE OPEN POSITION. VALVE WAS DEFERRED. LIGHT REMAINED ON IN-FLIGHT.

Narrative

MR X AND MYSELF COVERED THE TURN OF ACFT X IN FROM ZZZ1 AND GOING OUT TO ZZZ2. THE ACFT HAD A DISCREPANCY ON THE #1 ENG NOSE COWL ANTI-ICE VALVE. DUE TO THE CONSTRAINTS FOR TROUBLESHOOTING; DECISION WAS MADE WITH MAINT CTL TO MEL PER MEL 30-3-XX. I LOCKED THE NOSE COWL ANTI-ICE VALVE IN THE OPEN POS AND LOCKED THE HIGH STAGE VALVE IN THE CLOSED POS. AFTER THE ACFT DEPARTED; THE CAPT CALLED BACK AND REPORTED THAT THE LIGHT WAS STILL ON. I HEARD THE CALL ON THE RADIO AND THEN REALIZED I HAD FORGOTTEN TO DISCONNECT THE CANNON PLUG ON THE COWL ANTI-ICE VALVE AND ITS PRESSURE SWITCH. THE NEXT STATION WAS NOTIFIED BY MAINT CTL TO GO OUT AND DISCONNECT THE CANNON PLUGS. PLT REPORTED IT TO DISPATCH THAT THEY STILL HAD A NOSE COWL ANTI-ICE LIGHT AND DISPATCH RELAYED IT TO MAINT CTL. AS SOON AS I HEARD THE CONVERSATION TAKE PLACE I REALIZED WHAT I HAD FORGOTTEN. MAINT WAS NOTIFIED IN ZZZ2 TO MEET ACFT AND DISCONNECT THE CANNON PLUGS. I HAD LOCKED THE HIGH STAGE VALVE CLOSED FIRST. IT DOES NOT HAVE A CANNON PLUG TO DISCONNECT; I THEN LOCKED OPEN THE COWL ANTI-ICE VALVE AND TOTALLY FORGOT ABOUT DISCONNECTING THE CANNON PLUG ON THE VALVE AND PRESSURE SWITCH. THE LOCK-OUT PROC IS MECHANICALLY SIMILAR ON BOTH VALVES AND DUE TO COMPLACENCY OF LOCKING OUT THE HIGH STAGE VALVE I JUST PLAIN FORGOT TO DISCONNECT THE CANNON PLUG WHEN I LOCKED OPEN THE COWL ANTI-ICE VALVE.