how can i tell if it is time to replace the o2 sensor in my 93 del sol? it seems like the gas mileage is not as good as it used to be and a full tuneup has not really helped the mileage although it runs good.car is a D16z6 auto tranny and im getting around 240 miles on a 9 gallon tankful(about 27 mpg)is this about right or should i suspect the sensor is going bad?

 

There really isn't a way to test the o2 sensor, usually when they are bad you'll have a check engine light and 27mpg isn't bad especially if most of your driving is city....

 

Are we talking about a lot of city driving, at or less than 35 mph?

Otherwise, it could be a lot of things causing that kind of fuel consumption. O2 sensor not excluded.