The smog check station which said the CAT is not the problem is correct. The Catalytic Converter has no control on fuel delivery or air/fuel mixture.
A lean mixture could be caused due many things. You want to focus on possible vacuum leaks, defective oxygen sensor, defective fuel injectors, and/or defective ECT (Engine Coolant Temperature) sensor.
Your best bet is to have a smog check repair station conduct a smog check failure diagnosis. They typically charge two hours of labor to do this. It will cost you a little bit (an hour of labor usually costs $50-$60), but it will help you avoid replacing emission parts which don't need replacing.
posted by SmogTips Support
