Toyota Repair: 1992 Camry Smog issue, fuel air mixture, coolant temp

Question
This car is running rich.  Runs great but uses gas fast.  The haynes manual says its probably the O2 sensors.  I can only find the 2 sensors on the banks and I cant find a sensor behind the engine on the combined exhaust pipe.   this doesnt match the descriptions. Is this a unusual car?  not california or foreign as far as I know it was CA.  Whats the deal with O2 sensors and prices are some brands longer lasting than others? Does it make a difference to use universal sensors or does it matter that its engine specific?  Besides O2 sensor or a coolant temp sensor what would be the next likely culprit. I cant pass smog for high HC's

Answer
Matt,

I need a lot more info to assist with this question.
What engine is it? Auto or 5 spd? How many miles? What kind
of mileage city and highway, combination? Does it idle smooth?
Does it accelerate smooth? Does it start quick? Run any different
hot or cold? I need to know exactly what has been replaced and tampered with. Those sensors are there to tell the ECM the fuel
air mixture is slightly off. It DOESN'T mean the sensors are bad!


Tim