The model I got was about $150 plus shipping and it comes with a code book. It plugs in under the dash (and under the hood if you buy the adaptor), reads your code and the book tells you what it is.
It allows you to reset your "Check Engine" light and reset your green "Oil Change/Service Engine" lights back to 5 lights.
There are other code readers that require a laptop, require you to install software, etc., however, these are more expensive and do a lot more in terms of diagnostics and performance mapping. A few simple searches on this site and you'll discover plenty of other recommendations for code readers and reset tools.
just got a actron CP9180 and so far it works great, 1st time using a scanner tool, this has live data which I guess can be pretty useful if you know what to look for. got it from qualitytoolsforless.com
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