I have a 98 civic dx, which has two 02 sensors, one on top and another on the bottom. After bringing it to the shop, they told me that the reason y although one of my o2 sensor doesn't work, my gas milege is still normal is because the ECU is telling the it how much gas to use... which i have no idea about. Could it be because only one of my o2 sensor isnt working and the other one is still working properly so that my gas milege is still good? or is it really because of the ECU? Anyways, its been about 2 weeks of driving on the freeway now, do u guys think the 02 sensor problem might've messed up my cat converter already?

 

Is your CEL on currently? If yes, jump the service connector and get the 'blink' code - post that info back here.

But yeah if your rear O2 sensor, the one that lets the ECU know about catalytic converter health, was bad - you would still get decent gas mileage as its the primary O2 sensor that is responsible for fuel trim changes.

If the primary O2 sensor were bad, you would be getting worse gas mileage.