A B737-800 CREW WAS FORCED TO RETURN TO THEIR DEP STATION AFTER A MECH IMPROPERLY DEACTIVATED THE ENG ANTI-ICE SYS PER THE MEL.

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ON APR/FRI/02 AT APPROX XA00; A B737-800 OPERATING AS FLT ABCD TO LIT CALLED MAINT WITH A #1 ENG THERMAL ANTI-ICE FAILURE AND A WX FAULT. I RESPONDED TO THE CALL AND BEGAN TO WORK THE THERMAL ANTI-ICE PROB. ANOTHER ACFT MAINT TECHNICIAN SOON ARRIVED TO WORK THE RADAR FAULT. THERE WERE NO APPARENT LEAKS OR RUPTURES TO ANY OF THE DUCTING AND WITH DEP TIME APCHING; I DECIDED TO DEFER THE #1 ENG NOSE COWL ANTI-ICE SYS. MEL 30-XX-XXX (ENCLOSED) WAS USED. DISPATCH APPROVAL WAS REQUIRED AND RECEIVED. I BEGAN OPENING THE #1 ENG FAN COWL AND L THRUST REVERSER TO GAIN ACCESS TO THE NECESSARY VALVES. THE SECOND AMT COMPLETED HIS CHK OF THE RADAR AND BROUGHT THE MEL PAPERWORK. HE READ TO ME THE REQUIREMENT OF MEL 30-00-03B. THE MEL CALLED FOR 6 MAINT STEPS TO BE ACCOMPLISHED. STEP #4 WAS MISSED. THE ELECTRICAL CONNECTOR ON THE PRESSURE SWITCH REMAINED ON WHEN IT CALLED FOR IT TO BE REMOVED AND STOWED. ALTHOUGH WE WERE UNDER A TIME CONSTRAINT; I DON'T FEEL THIS WAS A FACTOR. I DO FEEL THAT HAVING THE MEL READ TO ME WAS. AS THE RELEASING AMT; IT WAS MY RESPONSIBILITY TO INSURE THE MEL WAS COMPLIED WITH. I HAD LITTLE EXPERIENCE WORKING WITH THE PARTICULAR AMT WHO WAS READING THE MEL AND RELIED ONLY ON A VERBAL INQUIRY OF 'DID WE GET EVERYTHING?' THE ACFT DEPARTED AND HAD A SUBSEQUENT AIR TURN-BACK. NO EMER WAS DECLARED AND NO PAX ANXIETY WAS RPTED. WHILE WAITING FOR THE ACFT TO RETURN TO THE GATE; I REVIEWED THE MEL REQUIREMENTS AND IMMEDIATELY REALIZED MY MISTAKE. WHEN THE ACFT ARRIVED BACK AT THE GATE I INFORMED A VERY UNDERSTANDING CAPT OF MY ERROR. WITH THE HELP OF A LEAD AMT; THE REMAINDER OF THE MEL WAS COMPLIED WITH IN A TIMELY MANNER AND THE FLT DEPARTED WITH NO FURTHER INCIDENT. TOTAL DELAY 1 HR 5 MINS.