Hi everyone,
Yesterday , running on highway on cruise control , car had problem ascend a little overpass ,
Roughly downshifted , and roughly upshifted after the overpass .
Went into trafic 1 miles away and car was shaking a lot before dying , restarted it and was really rough on idle , then died again , restart died ... , after 5-6 time restarted while giving gas on neutral, put it in drive and died instantly , never restarted that day
I checked the code today and got the following
P0141
P0303
P0327
P0332

Reseted the code and now only have misfire cylinder 3
Was missing a lot of oil , add 2 pints and started the car , stumble to start but idled not so bad , died 1 time restarted and was fine for about 10 minute before I stopped it
I went back 3 hours later pretty sure of a bad spark plug , started the car and it probably burned all the oil I added before
Checked the plug and got fire from it so not an electrical problem
I'm pretty good at mechanic but never touched a car motor
What could be the cause
Broken piston ?
Valve seal ?
I don't know what others parts are there so I don't know what could be broken

2003 lesabre limited

I wonder if your sucking oil via the intake.

Do you know the maintenance history of the car?

i would agree, sounds like the typical 3800 lower gasket or upper intake or both. probably get misfire codes and knock sensor codes when its trying to burn oil. the heated o2 may be nothing its one of those nuisance codes you reset and if it comes back there is really a problem

Sounds like the LIM Gasket to me as well, if you have never had the LIM Gasket replaced with the aluminum framed LIM Gasket, then that would be the first thing I'd do.
If you are unsure if the LIM Gasket is Nylon 6,6 or Aluminum framed, then you can get a multimeter, set it to ohms, and touch both leads to the edge of the LIM Gasket, if it reads OL then you have the nylon framed gaskets, and I'd get that changed immediately, in fact until you can verify if this is or isn't the issue, I'd recommend not driving the car anymore, unless you are willing to rebuild/replace the motor in a worst case scenario.

If you change the LIM Gaskets I'd recommend removing all of the plugs, pulling the fuel pump relay, and disconnecting the ICM wiring harness, then do a compression test on each cylinder, while the engine is still a bit warm, NOT hot as you do not want to burn yourself.

You'll want to change the oil, and do a coolant flush and fill when done.
If you need any links to help with the job, and any part numbers for the parts needed, let me know and I'll post those for you.

Thanks for the response
I really thought I was going to need a new motor or a serious rebuilt , gonna get the car to the a friend that is a mechanic and see if he get to the same conclusion as you

Gonna repost here as soon as I have news

As for the maintenance , got the car 2 weeks ago and it have 275000 km
previous owner admitted that he didn't do any oil change last 25000km
And it'* still my fault to have run 2500 km without even checking the oil level ...
I'm industrial mechanic so I know a lot about preventive maintenance

Very welcome, you may still need some work on the motor aside from the LIM Gaskets, but you won't know that till the plugs are pulled, and a proper compression test is done.
The lowest reading cylinder should not be less than 70% of the highest and no cylinder reading should be less than 689 kPa (100 psi). Perform compression test with engine at normal operating temperature, all spark plugs removed and throttle wide open.
With the amount of oil you are loosing, it very well could be leaking past the piston rings, so they may be damaged, or it just may be pooling up in the LIM.
The compression test will let him know what to do as far as if any rebuilding is necessary, hopefully a LIM Gasket job is all you will need.

Piston ring is in piece , and everything is scratched , destroyed the spark plug end ...
Motor also seized from trying to turn over without oil ,

So ordered a new motor should received it tomorrow

Is there anything I must know about swapping this engine ?
Is there a walk thought somewhere ? I didn't find any on google

Whatever isn't installed on the new engine, take from the old one and put it on the new one. EXCEPT gaskets.