4.9l V8xnh nuts and bolts

https://www.google.com/search?sa=X&source=univ&tbm=isch&q=2002+cadillac+seville+sls+engine+diagram&hl=en&ved=2ahUKEwi2hrmDsKfpAhUOfBoKHR5FDnMQsAR6BAgJEAE&biw=1326&bih=627

https://my.cadillac.com/content/dam/gmownercenter/gmna/dynamic/manuals/2002/cadillac/seville/2002_cadillac_seville_owners.pdf

Wow... Thats a lot of parts replacement for what may be a simple problem. The vehicle needs fuel and spark to start ( along with air and compression ).Is there spark from one or more spark plug wires when one is disconnected and held over a grounding point while the engine is cranked over? Is there fuel supply? Pressure to the fuel rail tested? Check the basics before you invest tons of money in an older vehicle

There are test strips you can get at auto parts stores to check for exhaust gas in coolant.

Not unless you want to cut a hole in the metal.
There is no access panel from the factory that I know of.

These instrument clusters were prone to fail !

autodata have a book of codes for all GM vehicles
you can get it on line from the autodata site

It is bolted to the throttle valve (where air hose from air cleaner connects to the engine) of the intake manifold, it is pictured below with yellow circle around it.

I had same issue. 1994 Cadillac Seville SLS serp diagrams not like my engine. Found that it is the same as 1993 Cadillac STS Northstar!
Try that, it worked for me. Good Luck.

how many miles are on it? if you have over 70k the head bolts may have let loose and allowed the head gaskets to leak they all do it

neither the 93 seville is just that it's the 4.9 liter before the northstar sts 4.6 came out in same year