Contract maintenance technician reported while accomplishing an MEL procedure for an avionics vent system fault on a A319 aircraft; they did not complete the procedure resulting in the aircraft failing to pressurize in flight.

Narrative

We are a contract maintenance facility for air carrier aircraft. I had been called out to troubleshoot a VENT AVNCS SYS FAULT illuminated on the ECAM. I am on call maintenance and mostly unfamiliar with any system on air carrier aircraft; or with the AMM (Aircraft Maintenance Manual). A contributing factor was this was a call that came in extremely late and I was somewhat fatigued. Working with maintenance control over the phone I expressed that I was unfamiliar with this system and with the AMM. The maintenance control told me we needed to complete MEL 21-XX-XXB. They sent me the reference. I was following along with the reference while completing the tasks that the maintenance control said needed to be completed; and trying to figure out if they coincided with each other. We opened/collard circuit breaker. Then we disconnected electrical connector and secured. I fully opened the skin-exchanger isolation valve with the manual mechanism; and pushed the CLR (Clear) button at the ECAM. This was what I was told by maintenance control was all that needed to be completed at this time; and that all we needed to do now was sign off paperwork to make the aircraft back in an airworthy condition. Being told that from maintenance control who knows these aircraft and associated maintenance manuals far better than I do; I believed that this was all that needed to be completed for this task. I did not read on at that point thinking this had completed what needed to be done. We finished up paperwork; I closed up and completed tool control. I went home tried to get some sleep then got called on the maintenance phone about three hours later with the same aircraft number that I worked on hours before; but this time it was for the aircraft not able to pressurize in flight. I headed back out to the airport to see what was going on with the aircraft. Once I arrived at the aircraft maintenance control had me start running test and troubleshooting. We came to the conclusion that the reason the aircraft would not pressurize was that the circuit breaker was still open/collard from the task we completed hours earlier. The current maintenance control individual I was working with now informed me that closing and uncollaring circuit breaker was part of the tasks that were supposed to be completed by myself and the help of the previous maintenance control individual. We read through the MEL and he showed me where it said to complete that task in the procedures. This was a scary and eye opening event for me to experience. I have now learned that even if maintenance control says this is all that needs to be completed; to verify myself that is actually the case and there is absolutely no further procedures needed to be completed.