While driving the transmission light came on the

Do you have any check engine lights on. There are a lot of different things you need to check, It sounds like you need to take it to someone who knows what they are doing.

If you changed the fluid, the level might be too low. Check level when engine is warm and running

this an auto box .if so check the transmission fluid level may be low .if so top up. if not is the fluid dirty .if yes.get a transmission flush.

check transmission fluid and is the check engine light on

It is the "Check Transaxle/Transmission" light.

Ensure your transmission fluid is at the appropriate level, as it should be checked at least once after any transmission service is conducted. Low fluid will cause the light to go on when you come to a stop and the fluid level moves forward, exposing momentarily the low level sensor.

If your level is correct and the issue is still occurring, there is a problem within your transmission and it should be brought back to the people who replaced your transmission.

The dipstick for a mercury sable is located behind the motor, and is a hard to see (usually black) circle dipstick. Do not mistake it for the oil dipstick located in the front, transmission fluid is red. The stick could also be, yellow, or red. Bring a light.

The fluid level should be in BETWEEN the cross hatch pattern on the metal stick when the car has not been running.

yes that is where you add CVT(continuously variable transmission fluid), I would check with the dealer before adding any regular ATF, make sure you have the right fluid before you ruin your transmission and are left with a hefty repair bill!!

disconnect battery for a few minutes then reconnect, this should reset the computer.

An easy solution is to make sure your not leaking transmission fluid and you have enough fluid in the vehicle.

This would be your engine check light. First off you need to check all your fluid levels. This includes windshield wiper fluid, brake fluid, oil, transmission fluid if it is an automatic. Remember that the transmission fluid on an auotmatic measuers in pints on the dip stick and not quarts even though it comes in quart bottles. The car must be in neutral and riunning while checking the fluid. Be sure you put the emergency brake on too just in case it ever slipped into gear. Things happen. Check radiator water level and power steering fluid.
With this light on it the computer will drop the engine into safe mode and have less power. If after all the fluids are filled and it is still on then it will be time to take it in and have the mechanic check the computer to tell you what needs repaired. This may cost 75 bucks or more but there is a thousand things that can turn that light on. Unless you can tell me if the car was acting up in any other features prior to the light coming on. If so then let me know and I will try to help you, Good Luck, The Raz

code p 1870 refers to transmission component slipping
That can be from dirty oil or a clutch pack or band slipping or low oil pump pressure in the transmission
you say that it occurs on highway driving after 2 hours and that slipping will add friction heat to the transmission oil bringing on the oil temp light
take it to an accredited transmission professional service center that works on your make of vehicle , have the oil changed which require the pan removal
at the same time that will allow the adjustments to be done
when reassembled and refilled , have a pump pressure test done