I Have a 98 Grand Cherokee with the 4.0, has 217,000 miles on it. A few months back I tuned it up, replaced the plugs, wires, cap and rotor. At this time it was running fine, it was just overdue for a tune-up. A few weeks after the tune-up it began running bad and the check engine light came on. I scanned it and got the error code 0171. I replaced the upstream O2 sensor and it seemed to fix the problem. A few days later it began running bad again, and began getting horrible fuel mileage. The check engine light came on and I got the error codes 0171 and 1281. I replaced the Downstream 02 sensor, the thermostat and the coolant temperature sensor. It ran great for a week or so, and the fuel mileage returned to normal. Now, its getting bad fuel mileage again and running rough. It seems to run even rougher as the engine warms to a certain level. Before I replaced the temperature sensor and thermostat this was also the case. I'm wondering if I should replace the cat or PCM. If anyone has any ideas I'd appreciate any help!

you could have some hard stored codes that only a shop can remove from your computer. obd-2 scan tools remove the codes on the dash. some stay stored but, do not reveal themselves. you could have an ignition system problem as well. the control module has symptoms like other sensors that can fool you. did you replace your IAT sensor? that's your intake air temperature sensor. it causes bad fuel mileage, increased starting time, and check engine light. you mentioned a few things that caught my eye. has your gas been running too rich or too lean? i had a bad IAC ( idle air control valve ) that caused my gas to run too lean. it fouled my O2 sensor ( upstream ) and my plugs. had to change them due to build-up on them. had a rough idle and terrible fuel mileage ( 1/2 of 1/4 tank of gas per 2 miles ) . your MAP ( manifold absolute pressure ) sensor can cause your gas to run too rich or too lean. as well as an MAF ( mass airflow ) sensor your vehicle has one. my 97 dosen't. check engine lights with both of them as well. do you ever get a PO351 code? that's primary coil secondary bank. my jeep loves storing that code. anything you move will set a code in your computer by the way. do you have strong smelling exhaust? surging in motor? hesitation? misfires or a miss? this helps to know. i suggest you log on to ehow.com and type in their search box symptoms of a bad ( you pick the part) and compare the symptoms listed against what you have been dealing with. that will help you narrow down your culprit. then come back for any additional assistance on here. it will help save you some time and money on parts you didn't need. good luck

Did you buy one of those generic O2's where you had to splice the wires?

check your acum lines from engine to exteriour parts i bet u have a bad or collapsed hose p0171 is a emission code not o2 so at least i can relate to this check your hoses and evap system i think youll find the issue