My 1996 Jeep Cherokee 4.0 is having issues running. It’s throwing trouble codes P0138, P0141 and P0171. About a week ago I was driving up a steep grade and it seemed the Jeep was lacking power. At the top of the grade the car started to stutter and there was a dramatic loss of power. I turned around and not far from the bottom of the grade the Jeep had a near complete loss of power with rough running. I could only drive 5 to 10 mph. The next day the Jeep wouldn’t even start. Since then I was able to get it running but it runs somewhat rough and bogs down just over 2000 rpms. There is a lot of black smoke and carbon coming out of the exhaust and it is burning way too much fuel. In the past couple of days I have replaced the spark plugs (the old ones were black from soot not oil), replaced the fuel filter, replaced the CCV fittings and hoses, removed and cleaned the throttle body, tested resistance on the injectors, removed and inspected/cleaned distributor cap and rotor (but not replaced), checked air filter and ran the car without it, unplugged the upstream o2 sensor and ran the car, and maybe some other things. As for the o2 codes I’m getting, I have had those codes up for years with no adverse effects. Years ago, I had a cracked exhaust manifold and had that welded and just inspected it as best I could without removing the intake manifold and didn’t see any cracks. Also inspected the rest of the exhaust. Any ideas of what’s going on?

I have not checked the Catalytic Converter. How does one do so?

Copy that. I don’t have that gauge but I’ll unbolt the system tomorrow and see what happens. I’ll post what happens tomorrow. Thanks for the reply.

So it did end up being a plugged up Catalytic Converter. I removed the upstream oxygen sensor and had a huge improvement in power and overall performance. Eventually I removed the Cat and gutted it leaving the downstream oxygen sensor unplugged when I reinstalled it. Not sure if that will mess with my performance or fuel economy but the Jeep is running pretty good now. Thanks.