MECHANIC REPORTS ABOUT A B767-300 AIR TURNBACK; AFTER E/E COMPARTMENT ACCESS DOOR EICAS MESSAGE CAME ON DURING CLIMB AND CREW UNABLE TO PRESSURIZE THE AIRCRAFT. DOOR FOUND LOCKED AND CLOSED.

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ACFT FLT RETURNED TO THE BLOCKS IN ZZZ1 WITH WRITE-UP: E&E ACCESS DOOR EICAS MESSAGE CAME ON DURING CLB AND UNABLE TO PRESSURIZE DURING CLB TO 5000 FT. PRIOR TO ACFT DEP; THERE WAS AN INBOUND GRIPE ABOUT LEADING EDGE SLAT ASYMMETRY MESSAGE WHICH REQUIRED ME TO ENTER THE E&E COMPARTMENT TO ACCOMPLISH A BITE ON THE PROX SENSOR ELECTRONIC UNIT. AFTER BITE TEST WAS ACCOMPLISHED; THE LOWER E&E DOOR WAS CLOSED; LOCKED AND CHKED FOR SECURITY BY MYSELF. ACFT PUSHED; TAXIED AND TOOK OFF WITH NO 'E&E COMPARTMENT DOOR EICAS MESSAGE.' THEN ON CLB MESSAGE APPEARED; ALONG WITH THE PRESSURIZATION PROB. UPON ACFT RETURN TO BLOCKS; AS SOON AS ACFT STOPPED; A MECH; ALONG WITH SEVERAL ARR RAMP PERSONNEL; WITNESSED THAT THE LOWER E&E DOOR WAS CLOSED; FAIRED WITH THE FUSELAGE; HANDLE IN THE STOWED AND LOCKED POSITION; WITH NO EVIDENCE OF ANY DEFECTS. THE ARR MECH THEN PUSHED ON THE DOOR TO SEE IF IT WAS UNLOCKED; AND FOUND THAT IT WAS LOCKED. HE THEN INSPECTED THE DOOR SEAL; FOUND NO DEFECTS; CLEAN AND IN ITS PROPER POSITION; AND THE DOOR SILL FOR ABNORMALITIES; FOUND NONE. HE THEN CYCLED DOOR SEVERAL TIMES AND HAD NO PROBS OPENING AND CLOSING IT. HE THEN PROCEEDED TO DEFER THE DOOR INDICATION. MECHS ALSO DID A PRESSURIZATION; DECAY CHK WITH NO FAULTS NOTED. THE NEXT MORNING; IN ZZZ; I CHKED E&E DOOR ON ANOTHER B767 ACFT; TRIED TO FALSE LATCH THE DOOR AND WAS UNABLE TO DO SO. THE DOOR DESIGN WOULD NOT ALLOW ME TO FALSE LATCH THE DOOR. WITH THE DOOR CLOSED AND HANDLE LOCKED AND STOWED; THE DOOR IS LOCKED. SO WITH THE INFO FROM THE ARR CREW IN ZZZ1 SEEING THAT THE DOOR WAS CLOSED; HANDLE IN THE STOWED AND LOCKED POSITION; WAS THE PRESSURIZATION PROB A PART OF THIS OR NOT? NO CODES FOUND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE ORIGINAL INBOUND GRIPE ABOUT THE LEADING EDGE SLAT ASYMMETRICALMETRY EICAS MESSAGE; INVOLVED THE NEED TO TROUBLE SHOOT THE PROXIMITY SENSORS FOR SLAT POSITIONS. THIS IS DONE BY PERFORMING A BITE TEST OF THE PROXIMITY SENSOR ELECTRONIC UNIT (PSEU). WHEN THE B767-300 LATER RETURNED FOR UNABLE TO PRESSURIZE AND A DOOR EICAS MESSAGE; THE RECEIVING MECHANIC PERFORMED SEVERAL TESTS AND INSPECTIONS. NOTHING SEEMED TO INDICATE A SPECIFIC CAUSE FOR THE AIRCRAFT'S FAILURE TO PRESSURIZE; AT LEAST WHILE ON THE GROUND. REPORTER STATED HE STILL DOES NOT KNOW HOW; OR EVEN IF; THE PSEU DID OR CAN; CAUSE THE AIRCRAFT TO NOT PRESSURIZE IN FLIGHT.