JSMEXUSA

Finally was able to get the code on my 99;read the stickies which says the following:
0702
Diagnostic Trouble Code 0702 points to a problem with the three-way Catalytic Converter.

The Cat is one of the principal components of the emissions control system. The Engine Control Module monitors the signals from the two front Oxygen Sensors and the one rear Oxygen Sensor. The signal frequency of each sensor indicates something about the condition of that sensor. A comparison of the frequency of the front sensors with the rear sensor indicates something about the condition of the Cat.

This malfunction was detected when the ratio of the signal frequency from the right bank front Oxygen Sensor to the signal frequency of the rear Oxygen Sensor exceeded a critical threshold value. This indicates that the Cat does not have enough oxygen storage capacity. The right cylinder bank is also called the rear bank. It is cylinders 1,3,5. Possible causes of this DTC include ...
- Three-way catalyst
- Exhaust tube
- Intake air leaks
- Fuel injectors
- Spark plugs
- Ignition timing

My car has 86000, almost 60000 with Bosh 4 platinum which were changed by previous owner; so I guess will start by changing them(of course with NGK either copper or platinum), then will check other possible problems; how do I know after changing spark plugs if that was the problem. I didn't clear the code from computer yet.
Can you tell me how to check the other problems: I am really broke and trying to avoid mechanic $.
- Three-way catalyst
- Exhaust tube
- Intake air leaks
- Fuel injectors
- Ignition timing
Thank you