My wife has a '98 Civic LX and the check engine light comes on "sometimes"...meaning sometimes when she fills up with gas her light comes on, or just whenever it feels like coming on I guess. The main problem is that every chance I get to take it to my mechanic for the test it goes out. He told me it was probably going to be the O2 sensor...not the cheap one. I had one I put in for like $75.00, but this one supposedly costs a bunch more than the one I replaced. Any suggestions? Plus she is getting really crappy gas mileage. Something like 20-25 mpg. Anything?

 

Grab some Sea foam too and use it to clean out the motor. Your local car place should have it. i.e pep boys or whatever.

 

sounds like it could be a evap problem replace your gas cap and check all vacuum lines

 

if you dont click your gas cap down three or more clicks the CEL light will come on. Try tighening the cap down more when you fill up with gas

 

I def think its the o2 sensor. Because of the crappy gas mileage. I dont know why one would be more expensive than the other. That doesnt make sense. The car is in limp mode because of the faulty sensor. It also has lst power because of the mode its in. I suggest you bag the new o2 and go out and get an 'o2 simulator'. You can get one at http://www.o2sensorsimulator.com/. Then have a midas or meineke install it for $50 at most.