Are you loosing any coolant? Before I did our Olds' LIM & UIM we would get those same two codes and could never figure them out. A new set of plugs always cured it for several months. After the new gasket - no problem. I was only loosing a little coolant, that I had mistakenly attributed to evaporation. Perhaps you could show us the what the plug looks like.

I lost maybe 1/2 " ,maybe a little more,but no more than 3/4" in coolant reservoir in 6 months. Most of it probably by the coolant level sensor. It'* allways wet,even after cleaning and allvays gives me false "Check coolant level" alarm.
The spark plug on #5 looked identically same as on #3 cylinder when I removed it.
Yes,you are right and I was thinking about it too but will try injector and ignition module first.
Thanks for suggestion. I woud hate to even think I need to change those gaskets.

I was browsing the forum and found this quote :
"A clogged Cat can cause a misfire..." .
Is it true?
If yes,can it cause misfire on specific sylinder?
Personally I doubt it,but asking just in case anyway.

Thanks in advance.

Well, if you are still looking for info on this, I'll throw in my two cents.

First, you ask if a clogged cat-conv can cause a misfire. While I've no doubt that the back pressure caused by a clogged CC could cause a misfire, it seems unlikely to me that it would be so repeatable in one cylinder. Then you have to consider what caused the bad CC to start with. Interestingly, two of the biggest culprits there are repeated misfires and a bad bank one O2 sensor. Both can create overly rich conditions resulting in the failure of your CC.

At this point, I would stick with assessing the injectors and go from there.

I believe it would be rare for you to find that the ICM was causing a misfire on one cylinder only, but checking it is not that hard.

And...not to be chicken little here....but you may very well be experiencing the beginnings of UIM and or LIM gasket failure. Some here have had extremely similar symptoms as thiers failed rather slowly. This can also lead to a clogged cat/conv.

Others experienced rather sudden UIM failure. Hard to say.

Get those injectors checked out and lets go from there.

I had a misfire on cylinder #6. Swapped plugs, wires, and coil packs with no change. Turned out to be the injector. Nbr 6 was the worse, but most of them were running at less than 90%.

I had them all "rebuilt" (cleaned and calibrated by a pro) for $150. Now it runs like a watch. I took them in to him, so that was a bench charge.

You can eliminate the electrical system as I did above easier than swapping injectors. So I would suggest doing that first if you haven't already. Remember if you swap the coil packs that you will confuse the cylinder numbers for the wires.

Also, if 1 injector is misfiring, you of course should do them all.

Thanks for info Lash and Bigfoot.
That'* a project for next week.This week I'm too busy with other stuff.
I will post info whether it fixed or not.

Regards

Ok.
Here is update.Finally swaped fuel injectors.Removed #5 ,put #3 injector in #5 cylinder and put one used but good in #3 cylinder .Still reading #5 misfire.
I have to assume those "famous" gaskets (either lower or upper) giving up.
By the way changed O2 and new Magnaflow CAT(old one rattled badly) and after pressing to the metall ( WOT )I smell sulfur. I'm tired of this car already :(
Any other ideas about misfire ?

Thanks in advance

Forgot to mention ,ICM was swapped too.