How do I reset the service light on a 1997 Volvo S90?

Not sure if it will work on a Volvo, but normally if the anti-theft system has been tripped just use the key to lock then unlock the door. If not, contact a Volvo dealer or local parts store for help.

there is a valve/switch on the ABS unit that gets dirty, this is probably the culprit. sorry no help other than that, but its a good starting point and it cured the problem on one that I had.

White smoke = water or coolant leak

Check any gasket loss or internal engine damage.

The service light comes on every 5000 miles to remind you to change the oil & check fluids. It is reset with a special tool that connects to the OBDII connector.

tune-up or possibly an O2 sensor. usually the check engine light will come on but maybe it's malfunctioning.

well, 'W" is for winter drive, "S" for sport drive and "E" for economy drive.

That code is for the engine coolant temperature sensor. You need to have a look at the connection to it, if it's good, replace the sensor. This is an important engine sensor, they are usually mounted somewhere near the front of the engine in the water jacket, exposed to coolant flow so make sure the engine is cold when you change it.

I would get the Starter Checked out. Sounds a lot like the Starter is not Catching on every Revolution Causing this Problem.

I can almost guarantee you that the gear position sensor is bad. I've replace 30 or 40 of these on 960's for these very same complaints! If you check the codes for the transmission, it will give a 3-1-3 or P0705 code. You will however need a code reader to check this on a 1997 model.
Check page 33 in this PDF for an explanation of the fault

Bret
RepairPal.com

local volvo dealer in Ft. Wayne called back and I gave them my vin and she gave me the code and it worked