How do you change the transmission filter in a 2002 Saturn SL1 1.9 liter engine? Can an after market filter be used or is it best to get a dealer part?

ask the dealer or , go there and see part for the car there
and see what transmission it is, for 2002 the move to 2003
and see if the same (gm parts)
there are 2 engines used, why post at all lacking such a key fact?
2.2L or 3.4liter, v6 is latter.and transmission vary buy engine max power torque, and more, gear counts for economy.

Oldsmobile Alero
or ask any engine rebuilder selling engine that (ever google that)
Or ask the shop doing it.
I'm there now. gm parts.
2002 , 2.4L 89059799 sku
fits all these

2002 Chevrolet Cavalier Base, LS, LS Sport 2.2L L4 - Gas 2002 Oldsmobile Alero GL, GLS, GX 2.2L L4 - Gas 2002 Pontiac Grand Am SE, SE1 2.2L L4 - Gas 2002 Pontiac Sunfire GT, SE 2.2L L4 - Gas 2002 Saturn L100 Base 2.2L L4 - Gas 2002 Saturn L200 Base 2.2L L4 - Gas 2002 Saturn LW200 Base 2.2L L4 - Gas
2003 there shows
89059802 , a diffr, sku!! shows fitments 03

2003 Oldsmobile Alero GL, GLS, GX 2.2L L4 - Gas 2003 Pontiac Grand Am SE, SE1 2.2L L4 - Gas 2003 Saturn L200 Base 2.2L L4 - Gas 2003 Saturn LW200 Base 2.2L L4 - Gas
that is the GM answer, others like engine builders/sellers
my say different even tell you what must be changed to do it

1 minutes work me. just reading at gm.com

It has a level overflow plug

https://www.justanswer.com/saturn/6x0z2-saturn-series-need-know-transmission-fluid.html

Checking Transmission Fluid - SaturnFans.com Forums

www.saturnfans.com > ... > L-Series Tech

Feb 21, 2008 - Checking Transmission Fluid L-Series Tech. ... I own a 2001 L200 Saturn and can't find where to check the transmission fluid ... Excess fluid will flow out automatically establishing the proper level and amount of transmission fluid. ... I lifted my 2002 LW200 4-cyl wagon on the lift with my mechanic but we ... L Series Auto transmission fluid change - SaturnFans.com ...
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Apr 25, 2010 - If the transmission fluid level is too high, fluid will leak out of the plug. ... There is a fill under the hood of the Saturn L200's I know for sure its in front of the ... I have a L200 2001 and the seller I bought it from changed the car from ... sounds good and let a mechanic deal with it at service time for the oil change. 6 answers

There are videos as well

https://www.google.com/search?safe=off&rlz=1C1QJDB_enCA643CA644&sxsrf=ALeKk03_q34GAPCNHHvd05BktURJ5V6TsQ%3A1608575709467&ei=3ergX8TQG4zr_QbSj6y4Dg&q=2001+Saturn+4+cylinder+l200+transmission+fluid+capacity

..

p1780 code indicates a problem in the transmission or the communication lines between the body control module and the engine control module...If the transmission had a true fault of this type in it, it would not cause the engine to shut down.
The most likely cause is that there was a glitch. Unfortunately this will be a reoccurring problem until it is fixed. If the engine check light starts to flash that means it is in limp mode and you will probably only be running about 10 miles per hour. I would say take it to Saturn dealer unfortunately they no longer exist. Best thing to do is have a transmission shop check it out. Or automotive shop sorry I don't have better news.

It could be and inspect your transmission fluid and have it serviced if it is the transmission fluid and also your engine oil. Engine coolant is corrosive to the bearings in your transmission and engine and can cause you to loose which ever you have cross contamination with.
If it's transmission fluid, then you'll loose your transmission is not corrected and if it's engine oil. Then your going to loose the engine shortly if not corrected.
Sorry for the bad news and thank you for using fixya.

First off an engine code 7 would mean it is a DOHC and an SW2, not SW1.

Do you have a dealer filter or aftermarket? The aftermarket have fitment issues (as well as flow issues) and it is best to use a dealer one.

Second, make sure the magnet is not on the filter area where it spins on. That can be removed during the first transmission fluid and filter change. It is only put on from the factory and removed afterward.

It is actually on the outside of the transmission on top parallel to the ground. It is toward the front of the car and is a large oil filter looking spin on filter.

There is no pan on an S-series car, simply a dipstick, external filter, and drain plug. Pretty much like changing the oil on your engine, actually easier!

Trying changing your transmission fluid and filter. The fluid gets gummed up and causes very rough shifting. It may also be low as well. I would suggest a full back flush where all fluid is pulled out of the system including the torque converter whereas a simple fluid and filter change will not do that. Also, there is a transmission fluid filter on the transmission lines, it looks very much like an oil filter. I believe that will greatly help your problem.

The 1.9 engine has a chain?

the filter for the newer cars contains the fuel pressure regulator so the filter is not the same.

I have a 2003 taurus with the same liter engine as your jag Ford has been making this engine since 1986 the only thing to worry about check the drive belt make sure transmission fluid has been changed and had all current oil changes and tune ups and also if I had found this deal before you did I would have a Jag :-)