1998 I30 with 180K miles..original o2 sensor. poor mileage

[infii30owner]

Hi..my 98 I30 with 180K miles appears to be getting poorer in gas mileage. what used to be 23mpg in city is now down to 18mpg. it still has original O2 sensor. I don't have any OBD codes or check engine light. Can bad o2 sensor contribute to poor gas mileage? If its gone bad would one expect it throw an error code?

sometimes i think if it ain't broken don't touch it...in this case if it needs to be replaced i better replace it rather than wasting away $$$s in gas. on ebay looks like i can get an O2 sensor for this car for $30.

also, i was afraid if my alternator was going bad. krazen folks said my battery is reading 13.8 volts when it should be reading 14. i have 3 year old 640amp battery from costco. a month ago, it acted up a few times during cranking up but it seems to be fine now. should i be concerned with alternator?

thanks.

[NutriaforBreakfast]

if they are bad they will throw an error code
When were the plugs changed last? Might check air filter, vacuum hoses,
tire pressure etc

[infii30owner]

then i'll refrain from touching sensors.

spark plugs were changed less than 2 years ago. have ngk titaniums. i'll check on airfilter. i usually make sure tires are at 32-34psi. don't know much about vacuum hoses.

thanks for your reply.