If you are on a budget, the is still incredibly powerful. It will scan every module in your car and tell you exactly what the fault codes are. You can then look those codes up on the Ross-Tech Wiki for free. If you need the full features, the best path is to:

If you genuinely just want to read and clear generic check engine lights on a pre-2006 car, the free, unregistered version might be enough. It will allow basic scanning and clearing of codes, even if it locks you out of Output Tests and Basic Settings.

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Ross-Tech (the developers) provides excellent forum support to registered users.