we will carry out leak checks(and approach the engine) with fan cowls open at idle only.
Any leak check under the thrust reverser cowls requires them closed to run the engines. Open cowls and carry out leak check after the engine is shut down.
Any leak check above idle(usually only high pressure fuel lines) require cowls to be closed for eng run, shut down engine. Open cowls after engine is shut down and carry out leak check.
Mistakes are made easily and can have devasating consequences, RIP Donald Gene Buchanan.