My CEL is on with code P0138 Oxygen Sensor B1S2 circuit high. I checked the voltage with my scanner and it reads 4.6 V constantly with cold or hot engine. What should the voltage be? I got the code right after replacing the O.E. style muffler with a Dynomax Super Turbo muffler, just coincidence? The front sensors seem to test ok and are switching.
Vehicle is a 98 Dakota 5.2 with 165k

 

I'd be tempted just to replace it with that many miles.

 

New O2 sensor fixed the problem. Old one was stuck on high voltage.

 

Glad it worked out for ya.