There is a very useful Popular Mechanics article on what to look for when buying a used car. One good idea is to take an OBDII reader with you so you can get the car to tell you the things that the seller will most likely neglect to mention; or on the rare occasion may not genuinely know.

I went and got me an inexpensive OBDII bluetooth reader. Which codes should I treat as show stoppers, which indicate possible problems, and which can I ignore?

 

Since you are asking for help on this, I would suggest if you find a car with codes showing take it to a mechanic to have it inspected before purchase.

 

take a reader, google the codes. usually P codes arent bad, sometimes gas cap, sometimes EGR valve or something emissions related. A codes are worse but nothing google cant tell you

 

Codes can easily be cleared. Unless the check engine light is on, wouldnt bother. Just take it to someone and have an inspection done.

 

https://www.s2ki.com/s2000/topic/659930-misfire-thread/

 

Codes can certainly be cleared before you arrive to take a look at the car. A OBDII scanner that can show 'live data' and determine if any of the monitors are in the 'not ready' state exists. The following video by scottykilmer provides further explanation: http://youtu.be/Rks40ng2C2Y?t=30s

I have this one and works okay and provides live data. http://www.amazon.com/Autel-MaxiScan...eywords=obd+II

 

It's unreasonable to expect people to list every possible code and the category that they fall into. Pull the code and look it up. If you can't tell if the code is a show stopper then get a trustworthy mechanic involved.