^ I agree. You need to find out what's triggering the CEL so you can work methodically. Without knowing the extent of the "maintenance" you or the mechanic did to clean the fuse box, it's hard to tell what went wrong or what connectors were moved/removed.

It would also help if you have access to your car's shop manual if the owner's manual does not have comprehensive servicing instructions.

You could also try disconnecting the battery for a few minutes and see if all it needs is an ECM reset (assuming the ECM memory gets erased when the battery is disconnected). Not quite familiar with Nissan ECMs.