Have a 99 S80. Keep getting the STC Service Required, ABS Service Required and Emission message. Dealer says that the ABS Control Module and Throttle module need to be replaced.

Reading the board it seems that the emission message may be due to a bad ABS module so I'd rather replace the ABS first and see if the emission message comes back.

Can I replace the ABS Control Module myself? What are the steps? I found a new module from out of state dealer for $600 while the dealer wants $800.

ABS controllers, like the one on your vehicle, have inherent design flaws.
The savings on the used unit may end up costing you more on the long run.
The mileage on your vehicle will give you an indication of the life expectancy on the ABS controller.
Find out what the mileage is on the vehicle that the used controller is coming from and that will give you an idea on how long the used unit will last before it fails (There is about a 90% chance it will fail around the same mileage as the one on your vehicle).
As far as removal, check my website, the top may be a little bit different, but the controller connectors and screws should be the same.
Victor's ABS Info
Click on the ABS/TRACS Repair

Disconnect battery cables in trunk. You can clear some messages by shorting the battery cables together while off the battery.

Remove the fuse box cover L (drivers) side back of engine compartment. There are latches to hold the cover in place. I believe there are screws to remove. Going by memory. Search the postings for ABS and S80 to get my previous instruction. Rotate the fuse box out of the way. The ABS module is held in place by long allen screws accessed under the ABS module. With the 4 screws out, the module will come off. The cable attaching the module to the fuse box will need removal.

Try the connector contacts first. Spraying with a contact cleaner is simple and worth a first try.

I opened my module (dremel tool around the edge) and inspected the soldering with a magnifying lense and found a solder bridge on the QFP, Quad Flat Pack , (Square shaped component) in the center of the board.

I resealed mine around the border with tape and it has lasted 4 months without the error message.