98 hx?? .....
frriennnd
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Welcome!
We have the same car! Even a CA spec too!
Unfortunately, getting good mileage means sacrificing things AC, which hurts your fuel efficiency.
As far as the 02 sensors go, I know that there are two of them on our cars. One is not too painful to replace, the other one is really expensive. But I could be mistaken. My advice is don't spend money to fix something that you are not sure is broken. Not passing smog can come from a lot of different things.
I don't know how many miles your car has, but I just got mine 8 months ago or so with 170k miles, and first things first, I ran "sea foam" through the engine which really help clear out a lot of buildup in the engine. I also changed the spark plugs and spark plug wires and that right there I KNOW helped out with the engine running smoothly and low end power.
I got performance 8.5mm plug wires, even though they were a bit pricey (80 bucks) but the new ones measured about 100 ohms and the old crappy ones on the car measured 10,000 ohms. Not exaggerating. So that maybe something like that will help you too.
... And another thing, our cars have "lean burn mode" that I do not believe the scan gauge will normally pick up on, but I think it sounds like there are ways to set it up to read it more accurately.
Welcome and happy modding!