97 xj 4.0 the question is does the o2 sensor mounted the the cat. affect fuel milage ? also do i need to have it at all? other than to keep the check engine light off. iam tired of replaceing the sensor. this makes 3rd one in a year. any help will be great.

 

The O2 at the cat monitors the cat, not air/fuel. If it's missing, the CEL will come on.

 

The O2 after the cat monitors the condition of the cat. Some newer cars if it doesn't get a proper reading in time it could effect the fueling of the car. Ie a richer mixture to up the temp of the cat to get it working properly. I know GM started doing this in about 02. But most cars it does nothing with the motor. Just turns the light on on the dash.

For one make sure the cat is good. Second don't use a Bosch O2 sensor. Those have gave people more issues then any other brand.

 

if your sensor keeps going bad after the cat, it is likely because the cat is not doing it's job, thus carboning up the sensor and rendering it useless. Sounds like you need a new cat bud.

 

thanks for the input everyone. i do need a new cat i know that. but the first o2 sensor craped out after i put the cat on 6 years ago. the second was a mud hole with sticks in it. this one works for a few days then the light come on so i just hat spending 70.00 on them.

 

check out www.rockauto.com, they have o2 sensors alot cheaper there... and like someone else already said, stay away from bosch. If you need a new cat however, it's going to keep throwing that code until you replace the cat.

 

well no codes just when i hook up my scanner. the front o2 is reading between .535 and .740 and the one in the cat is reading 1.000. it used to read between .600 and .740 before. so would say there is a problem and if i lightly tap on the cat it rattles. so got a new cat and o2 coming for it

 

Try the cat first. It is very possible that you won't need a new post-cat oxygen sensor considering the one you have is quite new.