2003 Infiniti I35 Shudder/Catalytic Converter Issue?

[ghettoceleb2679]

Greetings:

I want to start of by thanking all of you for your additions to this forum. Your insight has proved extremely valuable in helping me solve my issues. I just wanted to ask you all a few questions based upon your experience and knowledge. First, I would like to give you some background:

I own a 2003 I35 with 115k. I had it serviced regularly at my local dealer but around 15k ago (12/09) my extended warranty expired so going to the dealer for every issue is no longer an inexpensive option. Last time I was there they told me my cat was going bad. They stated trouble code P0430 was present and that the right front cat needed to be replaced and handed me an estimate for over $1000! I decided not to let them do the work because that was too expensive. Plus after doing some research (with your help) I found out that the cat should not just "go bad" on this model car, that something else had to cause it to go bad. That leads me to the H02 sensors.

The car had been running fine until recently. I was getting 25+mpg and the MIL would come on and go off randomly, so I just figured I was being railroaded by the dealer. Now the car starts fine but shudders big time at idle(no load) and when sitting at traffic lights (under load). It has never cut off or anything but the hesitation, loss of performance and drivablilty has me concerned. Now the MIL flashes and the trouble codes present are: P0430 and P0303(misfire cylinder 3). Some other codes that have been present before I took the car to the dealer back in December were: P1448 and P0455. Before the dealer inspected the car, I mentioned to them how my gas filler was no longer making the suction noise when I removed the filler cap and they supposedly tightened some vacuum hoses. Now the suction on the gas filler cap is fine and has been. Another note is: the car was running fine until I got a full tank of 91 octane gas (same gas/gas station I always use). Could this have something to do with the shudder/vibration? Now the car is parked in the garage with a full tank and I don't want to drive it because I don't want to cause more damage.

****Addition: My gas tank no longer has the suction sound when I remove the filler cap and there is this rather loud engine knocking coming from the rear bank of the engine (where cylinder 3 is located).******

My questions are:
1. Should I replace all of the H02 sensors before I replace the Cat?
2. Could a faulty O2 sensor cause a cylinder misfire and knocking?
3. If I replace all O2 sensors will I still need to replace the Cat?
4. Could a faulty Cat cause cylinder misfire?
5. How do I inspect for vacuum leaks?
6. Could all of this be caused by a vacuum leak?
7. What else should I look for/at to be the cause? I know you guys mention MAF and knock sensors but should I start there first or deal with what is causing the MIL codes?
8. Could this just be a case of bad fuel? And if so what can I do to remedy?

Feel free to offer any advice you may have that would help me with this. Thank you so much and thanks for the advice on the forum.

[MTZI30]

Have you ever replaced ignition coils? Theyd make the car run rough and would be hard on the cats.

[the converted]

I've was hunting issues like this on my car for a while. First things first, if you think you have bad gas, go and get some isopropyl alcohol and put it in your tank. If there is any water in there, it will help it blend with the gas.

Then looking at the 0303 code, that's the middle cylinder in the back that is showing a misfire. I usually would take the plug and the coil from that cylinder replace them with one from the front bank that is working well and then take the "bad" coil and plug and split them up. If the code stays in the same place, you have a wiring issue, otherwise it will move to the cylinder that has the bad component which would most likely be the coil.

Your 0430 code is for the front bank pre-cat being below threshold. It does occasionally happen that this is a bad o2 sensor, so being the price difference between the sensor and a new cat, I would try replacing the sensor first to see if the problem goes away. Since you have 115k on your car, it's about time that the o2 sensors can start failing.

Welcome to NICO, and let us know how things turn out.