The transmission may develop a harsh 1-2 shift and/or illumination of the Check Engine Light with code P1870 stored. This can be caused by wear of the bore for the TCC isolator and regulator valves in the valve body. This condition does not commonly occur until the transmission reaches normal operating temperature. If the valve body is worn, replacement will be necessary to correct this concern.Google+
Car Problem Reports
Chevrolet Suburban 1500 p1870 error code
Chevrolet Suburban 1500 Problem
Drive Trains Affected: 2WD, 4WD
Average Mileage: 122,264 mi (54,794 mi - 212,000 mi)
first time it through this code, had some other code issues and took care of them and then the p1870 code came on. didnt do nothing as of yet. just looking around to see what everybody else is trying. p.s my truck is a chevy 1997 4x4 1500
Visitor, 1999 Chevrolet Suburban 1500, 4WD
no fix yet, but research I've done shows that gm is aware of this and the fix is to have a shift kit put in. The metal for the part that actually does the shifting at different vaccuum points; wears out and has been fixed on 2000 and newer vehicles. I have not had the money to do this yet but there is a bulletin the dealer can pull up that refrences this trouble code.
Visitor, 2005 Chevrolet Suburban 1500, 4WD, 54,794 mi
Have this same problem and have taken it in many times to dealership who can't find a problem, turn off the warning light and then comes back on and happens again. What did you do about it and has it been fixed?
Drives fine until engine warms up; about 20 minutes, then shifts hard into 1st and 2nd. Turn the car off and start again it goes away for about 5 minutes and comes back again. Check engine light remains on.