Air Fuel and Oxygen Sensors - quick question

I don't have a check engine light but I've been thinking I should maybe replace my sensor(s) just to prevent any type of problems. The sensor before the cat was replaced about 2 years ago, the one after the cat has never been replaced. Looks like I can get them both for $177 here:

$137 Denso 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

http://www.amazon.com/Denso-234-9001-Air-Fuel-Sensor/dp/B000C5WD2Q

My question is: are these the correct one's and is Denso a good brand to buy?

2nd question: Will replacing the rear one do anything, or should I replace both?

 

denso is a very good brand.if the light is not on i would'nt replace them.

 

I've got the light . I just cant afford to fix them. What kinda problems could the sensors being out cause other than messed up mpg?

 

Also, if the Oxygen sensors arent working, the ECU is unsure of the current running condition of the motor and will TYPICALLY throw the vehicle into limp mode. ( revert to a presaved safer fuel/ignition timing maps. This usually results in running rich to avoid engine damage which = decreased engine efficiency and decreased gas mileage. )

Replace the sensor if you have CEL for an O2 sensor failure. Ive also seen people pull an 02 sensor and spray it down with WD40 or carb cleaner in an effort to band-aid it into working again.

If it aint broken, DONT FIX IT.

 

I'm confused about these sensors after reading all I could, is it only the upstream sensor that affects mileage? What is the function of the rear sensor, and why is it so much less expensive than the front one?

 

Typically there is a sensor pre catalytic converter and post catalytic converter.
Front usually monitors air/fuel mixture and tells the ECU to adjust accordingly. The back/post cat sensor usually monitors the efficiency of the cat to insure functionality to meet emissions requirements, and serve as another data plot to accurately shoot for a stoichiometric A/F ratio.

Its possible the rear sensor is comprised of cheaper materials due to not having to "sniff" directly unburned fuel/combustion by-products like the front sensor does and hence is cheaper.

 

the front is the air/fuel mixture.the rear monitors the convertor.so the ecm knows front o2 mixture,and then checks the readings from the rear o2 to check convertor efficiency.if your light is on,your ecm operates from whats called a reference voltage,which will get you by till the repair.(usually a little rich)

 

my CEL has been on for a week it threw the code for the front. I have the sensor (from rock auto) but havent had time to put it in. However, i am getting better gas mileage (about 2 mpg more) with the light on.pretty weird if you ask me!

 

wtf your getting better mpg with the CEL on?? well thats a new one,ive never heard of that happening before.

 

What would happen if your catalytic converter is not functioning properly?

 

I usually get about 300 miles to a tank. I am at 300 miles right now, and only at a lil less than a quarter of a tank. My driving habits havent changed or anything.

 

It wont pass emissions, will likely leave black soot all over your rear fender/ bumper, potential to foul the secondary o2 sensor.

Short of that, its function is for emissions plan and simple.

 

If it fouls the secondary o2 sensor, I guess that will make the check engine light come on?

 

Yes. Typically it would throw a CEL if the cat were dead as well as the secondary o2 would throw a "cat no workie" error.

 

I got the cel and pulled the code for the second o2 sensor, when i pulled the cat, it was empty, I want to install a test pipe and cat back exhaust, how do I get the cel to go out without a secondary o2 sensor?

 

^ yah that will do the trick,easy as hell install to. the code that it is meant to clear is P0420 IIRC.