O2 Sensor problem

[IBChillin32182]

A couple weeks ago my A/F ratio gauge started to respond slowly then quit working altogether. When i would first start my car the gauge would read all the way rich then slowly the voltage would fall till it read below the min voltage for lean. This takes about 15-30 seconds. after driving the car for about 15-25 min the check engine light would come on and I got code 91. I replaced both o2 sensors and it fixed the problem for a week. Now its doing it again. Could I have gotten a bad sensor or is it something else?

 

[90mustangGT]

Most likley, check for shorts though.

 

[MustangGlx]

Sometimes it's not the sensor that goes bad, but the connection that might be it.

 

[Michael Yount]

Have you run anything through the engine lately that might have taken out the sensors? Additives that aren't sensor-safe? Starting fluid? Done any repairs where gasket dressing, silicone, never-sieze might've gone through and fouled them? Burning lots of oil? Other things to think about. They shouldn't just go bad - given the environment they operate in, it's amazing to me they last as long as they usually do.

 

[GT40XStang9]

FYI:

When you start the car and they go full rich then slowly back to lean -- that's their normal operation of heating up.

 

[IBChillin32182]

I havent ran any additives through my engine and it doesnt burn oil it does have a leak though. I did get a little antiseize on one of them when i took the protective cap off of it but i wiped it off. Yesterday they started working again for about 10 min then quit. Today they worked for about 5 min then quit. Im beginning to think i have a short somewhere. Maybe where I spliced into the signal wire for the a/f ratio gauge.