If I read the FSM correctly, a code 42 is basically telling you that the ECM isn't receiving the information from the ICM it needs to take over control of the ignition timing. That could be caused by a bad connection to the ICM or bad wiring between the ICM and the ECM. Even though the engine may be running smoothly, it probably isn't performing as well as it should if the ignition timing isn't being determined by the ECM.