Oxygen sensor - one replaced the other ?

Check engine light on and the independent repair shop sez that the fault is with the oxygen sensors. Scheduled service. They said that one of the sensors was impossible to remove and just
replaced one. Truck seems to run fine but what am I missing here.

 

Forget to add that it is a 2003 4 cylinder with a manual trans

 

Really depends on a few things. If the code being thrown was specifically for the sensor they were able to replace, then sure. "impossible to remove" nope. Sure the stud nuts could be rusted on there, hell, the sensor could be rusted to the bung, but that just means you cut the bung out and weld a new one in. Certainly not impossible.

 

You need both sensors (upstream and downstream) working to run properly. They throw different codes. If its still throwing a code after replacing one you gotta swap the second one. Ive replaced both and they can be a pain. Nuts were rusted round. I used a small file and filed down flat edges and was able get a small wrench around it. The rear sensor (pic below) I swapped out at the same time when I put a new muffler in as I had it out.