99 Civic EX, New to me. Km per tank before replacing sensor 420 Km per tank after 530 Km. Nothing else changed except O 2 sensor. Watch for the check engine light and if it's O2 sensor, change it!

 

Probably a good idea to correct any check engine light as it normally runs the car in "limp" mode causing excessive gas consumption, loss of power, etc. But yes, sound advice to replace a sensor that is broken

 

when my front O2 went out my friend was going to junk a 99 malibu, he gave me the o2 sensor and I spliced it into my wiring harness, and has worked perfect for about 2 years now, and I get a min of 32 mpg city with normal driving habits and zero cel lights....