'Check engine' light is on. Auto technician ran computer diagnostic. He said the code showed the 'engine is running lean'. The fuel filter andPCV have been replaced, but we still get the computer code 'engine running lean'. No visible vaccuum lines off. Technician then smoke tested the engine. Two items showed visible smoke; The EGR valve showed very little smoke. The Idle Air Valve showed more smoke than the EGR valve. The tech suggested replacing the 02 sensor. Can the tech be right about the 02 sensor? Also, I am told there are FOUR 02 sensors on this car. (1997 Mustang GT) How can you check to determine which 02 sensor is bad? Please give dtailed response. Rather than just buying parts, I'd like to try to determine what part is actually bad. If you do not agree that the 02 sensor(s) are the problem, please explain why and suggest what I should do.