He needs to quit dawdling and check the tubes for leaks. You know, if he screws up and replaces the ECM when it is not the ECM, you do not get your money back for the ECM.
EDIT: The reason it comes on after a few days is because the EVAP system test takes place 6 hours after the car has been turned off. It is unlikely the ECM has a fault in it, that's NOT as common as an EVAP leak. I think your mechanic just doesn't have the stuff to test for an EVAP system leak.
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