Unfortunately you need a scan tool to retrieve codes stored in the computer of your car. The codes stored in the computer would indicate where the problem is or at least give a basic starting point to do tests.
If you get the codes stored (they will be in the form of P0420, or P1540) then you can look for test information for those codes. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online