mY ENGINE REPAIR SOON LIGHT CAME ON.WENT TO REPAIR SHOP AND WAS TOLD i HAD ERROR CODE NUMBER 1870. I WAS TOLD I HAD A PROBLEM IN MY TRANSMISION WITH MY TRANSMISION GOING INTO OVERDRIVE. I HAD NOT SEEN ANY PROBLEMS IN THAT AREA.THE CAR SEEMS TO RUN FINE.REPAIR WAS MADE REPLACING THE TCC REGULATOR VALVE ASSEMBLY AT A COST OF 424 DOLLARS.THREE DAYS LATER THE LIGHT CAME BACK ON WITH THE SAME CODE.WENT BACK AND HE CHECKED THE FLUID LEVEL AND RESET THE LIGHT.IT CAME BACK ON BEFORE THE DAY WAS OVER.HE TOLD ME HE WOULD CALL ME BACK WITH FURTHER ACTION.HAVE NOT HEARD FROM HIM YET.YOUR THOUGHTS ON THIS.I FEEL LIKE I HAVE BEEN TAKEN FOR A RIDE.I HAVE A 1996 CHEVROLET S10 BLAZER WITH 217,000 MILES ON IT.THE TRANSMISION WAS REBULIT AT 170,000 MILES.THE CAR HAS BEEN WELL MAINTAINED AND RUNS AND LOOKS GREAT.
I think that code is related to your torque converter so probably worth having a mechanic check into, if you like the truck. Just beware that a torque converter is the part between your engine and transmission, in the bell housing. The engine has to separate from the tranny to have them get to it – so it is a potentially expensive fix.
It could be a stuck or lazy solenoid as well, which is a less expensive fix.
…that, and your cap lock is stuck on. It makes stuff hard to read.
Question: If your truck “HAS BEEN WELL MAINTAINED AND RUNS AND LOOKS GREAT”, then why was the transmission rebuilt at 170K? Sounds like that was the bit that was neglected, rather than “well maintained”.
Chances are that you needed a whole new valve body as it is common in these transmissions for the regulator bore itself to wear. As such a new regulator won’t fix it b/c that isn’t the actual issue.
Here’s one of many links with some info on it: http://www.gmtruckhq.com/drivetrain/gm-service-bulletin-about-code-p1870-1578.html
The 1870 is likely due to a problem with a worn valve body bore and the circuit involving the TCC. I would think that if the transmission was properly rebuilt at 170k miles that the rebuilder woud be aware of this problem and would have resolved it during the rebuild. Or not. 
In an extreme case an 1870 can be caused by a major league engine hiccup involving the MAF sensor, etc. but this usually involves a bucking that would jolt you out of the seat.