Narrative:

I went down to aircraft to troubleshoot the captain efb inoperative. During the troubleshooting of that issue; the trim tank isolation valve pushbutton switch was flashing. I had this same issue only weeks before; and that problem was fixed with a relay. After suspecting the efb was a bad CPU; I quickly troubleshot the flashing trim tank isolation valve light. I got the relays I had replaced before; and it did not fix the problem. More troubleshooting would be needed. At approximately xa:00; [another technician] and I were doing more detailed troubleshooting and tracked down/replaced several other relays. With no fix yet; I got the lift truck and went up to the area just forward of the APU. [The other technician] cycled the trim tank iso valve switch several times; and no valve moved; or made noise. I had pulled up to MEL 28-22-54 on an ipad. We decided to MEL the trim tank valve. At approximately xb:30 I began the lock out procedure of the valve. Somewhere in this time; I had focused on one valve that I thought was the correct one. This valve was actually the APU low pressure valve; and 2 feet away with the same actuator on top is the trim tank iso valve. So; I actually locked out the APU from fuel. The trim tank iso valve was not locked out; but the circuit breaker was pulled and the system was not operable. At the time; I thought this was the correct valve and I was doing everything right. Looking at the area again today; I see where I made my mistakes. I mistakenly identified and was focused on the wrong valve. Many factors contributed to this event; including my self-imposed time limit of end of my shift; even though the aircraft would sit for another 7 hours; heat; MEL procedures; first time doing job; but mostly frustration. I think there were many factors that contributed to this event. Time constraints were self-imposed by end of my shift. Frustration level was high; heat and humidity were high; simple problem turned into hours of troubleshooting. To prevent this from happening again; if nothing is working; slow down and step back. Take a break and re-assess the problem again. I did not!

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Original NASA ASRS Text

Title: Maintenance technicians described the conditions and situation that contributed to locking out the incorrect fuel valve while deferring it per the MEL.

Narrative: I went down to aircraft to troubleshoot the Captain EFB inoperative. During the troubleshooting of that issue; the trim tank isolation valve pushbutton switch was flashing. I had this same issue only weeks before; and that problem was fixed with a relay. After suspecting the EFB was a bad CPU; I quickly troubleshot the flashing trim tank isolation valve light. I got the relays I had replaced before; and it did not fix the problem. More troubleshooting would be needed. At approximately XA:00; [another technician] and I were doing more detailed troubleshooting and tracked down/replaced several other relays. With no fix yet; I got the lift truck and went up to the area just forward of the APU. [The other technician] cycled the trim tank ISO valve switch several times; and no valve moved; or made noise. I had pulled up to MEL 28-22-54 on an iPad. We decided to MEL the trim tank valve. At approximately XB:30 I began the lock out procedure of the valve. Somewhere in this time; I had focused on one valve that I thought was the correct one. This valve was actually the APU low pressure valve; and 2 feet away with the same actuator on top is the trim tank ISO valve. So; I actually locked out the APU from fuel. The trim tank ISO valve was not locked out; but the circuit breaker was pulled and the system was not operable. At the time; I thought this was the correct valve and I was doing everything right. Looking at the area again today; I see where I made my mistakes. I mistakenly identified and was focused on the wrong valve. Many factors contributed to this event; including my self-imposed time limit of end of my shift; even though the aircraft would sit for another 7 hours; heat; MEL procedures; first time doing job; but mostly frustration. I think there were many factors that contributed to this event. Time constraints were self-imposed by end of my shift. Frustration level was high; heat and humidity were high; simple problem turned into hours of troubleshooting. To prevent this from happening again; if nothing is working; slow down and step back. Take a break and re-assess the problem again. I did not!

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.