This is a pretty simple tutorial, which will tell you what you need to do to fix the Damaged engine bug that plagues a number of vehicles in CoH. This tutorial requires you to have and be familiar with Corsix's Mod Studio, especially with adding requirement actions.
1. Find the vehicle engine damaged and destroyed criticals.
2. In each one, open the requirements table and add a required_critical action. You'll need a required_critical action for each critical that occupies the same "critical slot", i've organised the listed criticals into groups so it might be easier to understand what i mean. For each requirement action, in the 0x7AB17C06 field (on my Mod Studio it's an unknown field) enter the path to the critical you want required (eg. critical\vehicle_damage_engine_incremental.lua). Set is_present to False, and set the reason to usage. Do this with additional required_critical actions in the requirements table, so that the critical requires that itself and the related criticals that would occupy the same "slot" not be present.
In the vehicle_damage_engine critical for example, you'd have 3 required_critical actions, one for itself, and one for each of the other two criticals that are in it's "group". Save the file, and do the same for the other criticals i listed above.
And that is pretty much it. Now any vehicle can only have a damaged engine or destroyed enging critical once, and it cannot get it again until the existing one is removed by repairing the vehicle.