running rich, code 42, does not switch

[nothinbutroad]

code 42-exhaust gas oxygen(ego) sensor: voltage signal always "rich"(high value)-does not switch.

so i have nearly fixed my 91 explorer, all thats left is a code 42. i replaced nearly all my sensors(including a brand new 02 sensor), did a compression test on the engine, and my obd1 scanner came back with a code 42 during a koer, and passed with an 11 in the koeo. i have a smog test coming up in august and i need to fix this problem. what are some causes for an engine running rich? code 42 also says (does not switch) what does that mean? what would i need to do to make it switch? or what would be the first thing i check to narrow down my diagnosis? i greatly appraciate all help and advice.

[nothinbutroad]

i have read a few articles about the fuel pressure regulator contributing to running rich. could this maybe be my last part to change out before im good to go? before i changed all my senors i experienced jerking during acceleration, and stalling while at a light or stop sign. i no longer have jerking but when my rpms drop(even at idle in park) the engine chugs and acts like it wants to die but doesnt quite kill the engine, just barely. am i close or way off?

[MrShorty]

code 42 also says (does not switch) what does that mean? what would i need to do to make it switch?

In normal operation, the O2 sensor signal is supposed to continuously swing from "rich" (>0.45 V) to "lean" (<0.45 V). "does not switch" means that the O2 signal is not crossing that ~0.45 threshold value, and "rich" means that the signal is staying on the high side of the threshold.

1st thing I'd probably check would be the fuel system. Check for gas in the vacuum line to the FPR, which indicates a ruptured diaphragm inside the FPR. Then check the fuel pressure and make sure it is within specs.

[nothinbutroad]

that was a great help thank you, im about to replace the fuel filter and i have a fpr being shipped in.

[nothinbutroad]

so i replaced the fuel pressure regulator today, it defiitely helped some other problems, the engine doesnt chug when i let off the gas and it doesnt wanna die, its smooth when i stop and theres no hesitation or jerking when i hit the gas, and when i scanned it i got an 11 koeo and 11 in koer so thank you so much. but i still have the the problem with my back fire. would the large crack(split/hole) in the welding of the muffler be making the popping, backfire noise when i put it in drive? or something else?