My celica has been getting poor gas mileage lately.

So I decided to do a tune-up and replace the oxygen sensor.

Does anyknow know the right Ohm reading the sensor is suppose to have? Should i replace both of them, the one before the cat and after?

I would just change the primary 02 sensor (before). Then second one is just a slave to the first, to just verify readings.

For the price of a new sensor at an auto parts store, you might as well spend a little more and get an OEM one from Toyota.

how much does toyota charge?

get a denso from an online retailer, same part toyota sells. denso first time fit.

I'm curious about this o2 sensor replacement, is this something that should be replaced every XX miles? Or do ya wait till the CEL light comes on? How might one know that it's time to replace it?

O2 sensors need to be replaced every 75,000 miles to maintain optimum gas mileage performance.

I'm noticing poor gas mileage and my 7afe which has about 90k on the clock so its about time to do the change... I'm just too cheap to deal with it. But most people wait for the cell light to come on before replacement, which is understandable because the sensor can cost around $50- $150.