Heres the deal: I have a 99 civic EX 4dr.
After i replaced my primary oxygen sensor wit ha Denso universal one(its wired up correctly, CEL doesnt come on anymore) and iv been getting 30 mpg consistantly through the last 4 fillups. I used to get about 35 mpg. Also what i did was put fuel injector cleaner through my system also.

People say to change the sensor for an OEM one but what im wondering is, is the primary sensor more "important" than the 2nd one? if so i am wonderinf if i can switch the catalitic converter sensor with my primary universal sensor till i am able to get some cash? will that make a difference or what are peoples thought on doing that?

 

The primary is more important than the secondary... I dont think you can switch them though, I think they use differnt connectors and amount of wires.

 

They are in fact interchangeable - the only differences between the two are in their construction, as in, length of wires and the inclusion of heat shielding.

Yes, the primary is more important than the secondary. However, since you've replaced the primary, then that probably means you have as many miles on the secondary. If thats the case, then trying to swap the high mileage secondary for the new primary Denso universal would probably net you worse gas mileage.

 

the upper can be moved to the lower and vise versa the harnesses are the same. i know for sure

 

I just went to the gas station to get a lighter that was only $1....i took my old sensor and was just messing around with the sensor hooked to my VOM to see if it would give off any voltage from the lighter and then i had freetime so i just poped it into my car and drove it around for about an hour and no CEL anymore...maybe i cleaned it off enough for it to work again. i will fillup tomorrow and reset my trip. and then see how that goes

 

Well dont forget every time you reset your ECU it has to learn the fuel curves. Long term and short term, it takes a few drive cycles for the ECU to compute these caculations to give you the best fuel consumption. PLus right now gas is being converted to Summer fuel so its going to be less oxidized and in some cars the fuel economy is going to go down.