An A321 air carrier technician reported moving the door operating lever to the open position while troubleshooting the main entry door escape slide arming lever. The escape slide deployed.

Narrative

While troubleshooting a cabin write up for the L4 door mode arming control lever; the emergency slide accidentally deployed. The flight attendant had written up a problem with the L4 door arming lever. I cycled the mode control lever to verify the problem and noticed it was not centered. Per the AMM (Aircraft Maintenance Manual) it said to verify the girt bar fittings for debris. I deactivated the lever opened the door once about 4 in. Inspected the floor fittings to be clear of any debris. I then closed the door and cycled the lever again to arm. Nothing changed the lever was still off center bordering the red and white markings. Then I decided to check the girt bar itself for debris. I know that I deactivated the mood select lever but that part is now foggy and I cant say for 100% it was disarmed. What's clear in my mind is what happened next. As I went to open the door once more I heard the rush or air coming the cylinder; and the outward pull of the door. I immediately thought to myself '[expletive]'. I then released my grip of the door handle and saw the slide activate and inflate in front of me.I should have slowed down and not been in a rush. I should have taken at least 5 seconds and looked at the handle to verify 100% it was in the disarmed position. I should have had at least a second person verify my actions and that I was in the correct sequence of arming and disarming the mode select lever.