So the gas mileage in my '98 XJ 4.0 has kind of went out the door so after some research I decided to change the upstream O2 sensor. After changing to an NTK sensor, I now have a CEL. Took it in to have it read and it kicked out a P0306, P1388, and P0303 code. It says the 303 and 306 code are misfires. The 1388 code is Auto shutdown relay control circuit malfunction. No CEL before changing the O2 sensor and now 3 codes. Coincidence? Thinking I'll go to NAPA and buy some plugs and wires today. I just don't want to be chasing my tail so any help would be greatly appreciated.

My understanding is you should replace both O2 sensors at the same time. Reason being if the info from the two sensors is out of spec from each other it can trip codes. HOPE I explained that right. I changed both when my mpg tanked and it solved the problem.

Replace the downstream w/ an NTK also and clear the codes.

Forgot to add that the idle is much rougher now too.

And that's probably due to only replacing one sensor. As EZ and I have said, replace both and clear the codes.

Let us know how it works out after that. It should be fine.

Just got back from NAPA. This afternoon will be spent replacing downstream O2 sensor, plugs, wires, cap and rotor. I figured while I had the hood open I'll just do it all. Stay tuned.

FAIL!!!! (Sort of) Can not get rear O2 sensor loose. Tried spraying it, tried running the engine until exhaust was hot and it won't budge. Rounded it off a little trying to break it loose. I did however change plugs, wires, cap and rotor and reset CEL. Runs and idles much better and so far no CEL. Not sure what to try next on the O2 sensor.

I had to have one of them torched out and tapped when I changed the 4 on our Blazer. All came out fairly easy except for the last one of course. Downstream one by the way.

Awesome! Hang in there, someone will chime in to help. I'm at a loss and really don't know how to help. Never had that issue living in the south...

So, it's running fine? No CEL?


Done.


I wish I could remember who it was, but maybe a year, year and a half ago, someone posted the section of the FSM that said that the downstream O2 sensor is ONLY for telling the PCM when the cat needs to be replaced, it has nothing to do with running the engine.

Big pipe wrench with a 4 foot breaker bar. Once it bites it will rip it out. Make sure you get the jaws on the hex head. I just did one that took the threads with it so I had to get an 18 x1.5 tap to recut them. I used eBay to find an Irwin 2459, $12 shipped. Oxy/acetylene and wd40 work wonders as adjuncts to the pipe wrench.

WD-40 is about the sorriest excuse for a penetrating oil on the market. It wasn't originally marketed as a penetrating oil, actually. It's a Water Displacer, and works well for that.

PB Blaster is far better, as proven in controlled tests. Kroil is better yet, again, controlled tests. The best is a 50/50 mix of acetone and transmission fluid. Same controlled tests.


I've been using Seafoam Deep Creep lately, and it's not a controlled test, but it has seemed very effective.

You are correct; I was in error re: the codes. It is recommended that O2 sensors are replaced in pairs though.

The downstream oxygen sensor is used to monitor the catalytic convertor. As the substrate inside the convertor deteriorates, the input from the downstream oxygen sensor 'should' match the upstream oxygen sensor input, with a slight delay. By comparing the downstream and upstream sensor inputs, the PCM calculates catalytic convertor efficiency. When it drops below emissions standards, it trips the CEL and stores the appropriate code. That wasnt word for word, but is basically what the FSM for my 97 has in it. (Group 14-30 Fuel System).

If the CEL is off and is running much better, id leave the rest of it be for now.

Wd40 is not being used as a penetrating oil in this case. Torch to heat it up, wd40 to cool it. Cycle it a few times and break it loose.