iEngineCheck is an app in Cydia that allows you to check what the engine light on your car means. Upon launching the app you will need to select your cars manufacture and then the model. Once you do this you will be taken to an overview with the various dashboard-lights, if you click on one of these lights you will be taken to a page with more information on that specific light. There are plenty of apps in the app store to accomplish this, and this app doesn't have a lot of manufacturers listed, yet.

Credits to iJailbreak for the picture.

This tweak is available for free from the BigBoss repo.