GMC Class 4 (continued from page 46) 159.5-, and 183.5-in. wheelbases. ... According to Hopkins, the simplified Standard power for the Sierra 3500 HD will be the 5.7-liter EFI gas engine, but the Mark V 7.4-liter EFI ... Service is simplified by a built-in self-diagnostic LED display that signals that a failure has occurred and directs the mechanic where to look for ... The diagnostic memory is retained even if the system is disconnected, and can be easily reset with a common shop magnet.