coldbloodrain

i have a 96 xj 4.0 and i have the check engine light on running codes p0171 and p0172. how would this make the jeep run (like would it make it seem like im having other problems) and how do i fix this?

shaner517

I had the same Po171 code in my 96. If remember correctly its a fuel running rich code. I replaced my upstream O2 sensor and it went away.

kjc

po171 and po172 are rich and lean mixture correction codes.. it sounds as if it's detecting a too lean mixture and overcompensating which causes an over rich condition. My guess would be a new 'up-stream' O2 sensor. They wear out over time, and many people don't replace them regularly as recommended. The O2 sensor sends a signal to the PCM which uses this information to adjust the engines other components, like fuel injectors etc.
*Another guess would be a fuel injector problem (stuck open or shut) Check your fuel pressure at the rail is up to par before blaming the injectors tho.
*That's the first things I can think of, I'm sure others will chime in with more.. like general tune-up, check ignition coil(s) battery fully charged etc.
*Also, some codes are set due to a temporary condition (a misfire for example) The first thing I'd do is re-set the codes and see if they return. Hope it's something simple! Let us know what you find out.