how can I repair my car, the RPM goes up and down, wastes alot of gas. on 1993 Toyota Camry

the check engine light was on. i changed the coolant/temp sensor, and the engline light turned off. But, the RPM gauge still has a malfuncion, it goes up and down. There is strong odor of gas and the car wastes a lot of gas. It has not passed smog. I took to a shop and they claim to have changed the O2 sensor, but this one looks very fragile/small. The check engine light turned on about 20 miles from the shop. I was not happy with their service. They told me to take it back, but I really don't want to. I want to fix the car myself. I've had bad experiences with mechanics/shops. please help.

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2 answers
the only tells you which circuit has issues not the sensor. what is the code?
You want to fix your car yourself? O-kay...first I would try to solve your "check engine" light...you will have to jumper the DTL to get the code(s).