Engine light on, codes are PO300, PO302, PO305. what do these mean?

Do tune up,coils module,wires,plugs and see what happened..

you have multiple misfire po300 and cylinder 5 po305 misfire.the misfire is causing the rough running.The computer has disabled your traction control.You either have carbon in the EGR which simulates a vacuum leak,bad spark plugs,a leaking intake manifold,or low fuel pressure from a bad pump or clogged filter.
i would start with the spark plugs and fuel filter

PO300 and P1399 are random cylinder misfire detected.
PO301 cylinder 1 misfire
PO302 cylinder 2 misfire
PO304 cylinder 4 misfire
PO306 cylinder 6 misfire
I would say that you have damage to the ignition wiring harness or you ECM is failing.

If a tune up (spark plugs, ignition wires, distributor cap/rotor button or coil packs) don't work, try checking the fuel system.

Which code you have po300 or po301.po302 .PO301,PO302 will be coil or module.PO300 could be bad spark plugs,vac. leak,clog fuel filter,bad fuel pump,bad battery.

We can't help you here

You need the car diagnosed

I doubt you have lifters,should have
OHC valve train

Most likely the oil passages are clogged up
in the cams with sludge

That's what happens when the wrong oil
& too long between changes takes place

You ruin the engine

2002 Envoy sle 4.2 L straight 6. Random misfire(PO300). After erasing codes for several days, I finally took my car on the highway and ran it at high speed(70+) for about 4 to 5 miles. Check engine light came on as before but this time it gave me(PO305). Random misfire cylinder 5. So I switched coil 5 with coil 2. Went back out on the highway and repeated as before. This time it gave me(PO302). This was telling me I had a bad coil. Replacing one coil pack solved the problem. The professionals told me low fuel pressure would not give me random misfire, but rather a O2 code because the engine is running lean. Thank you------------------------------------ thank you very much.

These are misfire codes.This means that your vehicle is missing.The 300 is a random misfire and the 305 is for cylinder 5,and the 306 is for cylinder number 6.Check the plugs if they are good then you could have bad coils.They are top of the plugs.Hope this helps.Good luck.

Hi these are mis-fire codes ,
P0300 is a general mis-fire detected code the others relate to specific cylinders,
P0302 is cylinder 2 misfire
P0303................3........
p0305..................5........
I would check your spark plugs first if Ok you need to make sure the coils are working ok for this you need an osciloscope to check output voltage if your not sure then take to a qualified mechanic with Cadilac experience it could be caused by an air leak,bad ECT sensor or even Mass air flow sensor ,codes only tell you the result not the cause

Try some scrapdealers. They keep bits they think they can sell from crashed/scrapped cars etc.