2003 jeep liberty idle miss fire codes remove exhaust runs now

run the fault codes. check the serviceability of the cat converter as it may be broken inside and blocking the exhaust

backfiring is an indication of incorrect spark firing order or timing check that the HT leads are not touching each other and that the firing order is correct

a misfire code , could a number of things ,bad plugs , the number one thing is the cat is going bad get the codes and get back on here and post them

multiple code issue (evaporative, catalyst efficiency, and miss fire)doubtfull all 1 source of error but draw attention to the miss fire code PO300 is a random cylinder miss fire, possible crank position or cam position sensors.but i would check timing first.

po300 is random miss fire doubtful two O2 sensors bad at one time most likely coolant temp sensor is bad causing engine to run rich this will cause O2 sensors to throw a code(thats what they are there for..exhaust gas temp. and raw fuel) running rich will also fowl spark plugs(=miss fire) P1128 ??

Drop the tank remove all fuel replace the filter and fill with fresh gas

The code 1296 may be the MAP sensor which measures intake vac, and the code 301 is for a miss on number one cylinder.
Check and see if you got everything connected back up.

this is a code for misfiring ( cylinder #4). you need to reset engine computer first. you can do this by removing negative cable from battery and waiting for 15 minutes. if there is save your radio code.

if it returns back, faulty cable for spark plug, old spark plugs, coil(s) may cause this. replace spark plugs first(it is the chepaest fix). mostly you can cure problem by this way. if not your oxygen sensor can cause this. you can understand this by running engine and observing how it idles and how it smells at back. if it start to idle rough and start to smell bad ad exhaust pipe. o2 sensor is gone.

you need to proceed step by step. 1 variable at a time please.

in extreme cases burned exhaust valve,or broken ECU can casue this too.(very rare)

Sounds like you may need a fuel/air mixture adjustment, this is quite common with higher altitudes than what your vehicle is normally running in...

Time to check sensors. I'd start with tps, map and mass air flow sensor if used. Many "supermarket" style parts outlets will scan your system for codes. Start there to give yourself some general direction on what to look at.(of course you will check ignition components, cap, wires, rotor and coil... believe it or not, even if there is residue around plugs, it generally won't give you that problem, though it should be blown out of there with compressed air or something before removing plugs)