A Line Mechanic reports he failed to visually verify an avionic's ground cooling valve was in the 'closed' position on a CRJ-900 as part of an MEL deferral of a PAX DR OUT HNDL EICAS message. Aircraft returned to airport due to unable to pressurize. Mechanic had unknowingly repositioned valve to the 'open' position.

Narrative

A CRJ-900 arrived at ZZZ. On checking logbook; pilot had written up PAX Door OUT Handle caution message. Following MEL procedure 52-70-1-A; item (i) refers to avionics cooling is considered inoperative. I then came in and printed out the procedure for deactivation of ground valve; Task 21-24-18; (configuration XXX). Following the task for the cooling valve; I removed the carpet and round panel under the carpet located on right side of aircraft; just forward of emergency exit; approximately seven screws. Reached down through the cables and disconnected the cannon plug; capped and stowed [canon plug] on the bottom of stringer area in the insulation. Then reaching up; I rotated the valve handle to the opposite position. Re-secured the panel screws; installed rug. Finished MEL / placard procedure; logbook placards. Dispatched aircraft. Maintenance Control called as to [aircraft] returned from flight. [Aircraft] could not pressurize. On return to gate logbook entry. This return from flight occurred in that I did not visually verify the the position of valve handle. Having done the procedure before and the valve had always been in the 'open' position; I reached through the cables; turned the valve [but] not verifying position. When the aircraft returned and I removed the panel and reconnected the cannon plug to figure out what I did; the valve moved to the 'closed' position; so it had been in the 'closed' position on arrival and I had positioned to the 'open' position. I visually verified this time; plus pressurized per AMM 21-31-00 this time; no problems; dispatched aircraft. I cannot say the problem was aircraft or procedure. I should have visually verified position of valve handle.