91xj

My jeep is a 91 with the 4.0. I get code 21 when I cycle the key. I have replaced the o2 sensor twice already with no luck so I'm guessing that can't be my problem. When I start it up and start driving it sometimes sputters like its going to die but it doesn't. This usually only happens once until I shut it off again. I have looked all over online and have found similar issues. Some say tps, others say ignition coil, and even the pcm. I would rather not just throw parts at it. Any help is appreciated. My gas mileage is suffering a bit as well.

tjwalker

Always follow the code. If you continue to get a code 21, concentrate on the oxygen sensor CIRCUIT. When I say circuit, that means along with the sensor itself, the connector for the sensor and also every inch of wiring. Not unusual to see wiring issues related to oxygen sensors. Inspect it carefully.

Some other things to think about....

1. The oxygen sensor is heated and is fused. Check all fuses in the power distribution center (black box under the hood). This fuse I believe is a mini 15 amp.

2. What brand oxygen sensor are you using? I recommend NTK; never use a no-name sensor.

3. Check your exhaust manifold for cracks/leaks......

91xj

I'm using a bosch. I have inspected the wiring visually all the way along the fuel rail. I have not hooked a tester up to them, but all looks good. I will have to check the fuses. I didn't know it was fused. I have heard that Bosch is not a good brand for o2 sensors on jeeps, but they have a lifetime warranty from autozone so I got a second one free thinking the other was bad. There is a very small exhaust leak because I went cheap and got a pace setter header that has slip joints with clamps. I welded as much as I could with it installed. I don't think it could be leaking enough to cause an issue as it is very small. Thank you for the reply and I will check my fuses

jvilas

Any luck?
I have pretty much the same problem. Bosch o2 sensor and still getting code 21