Vehicles equipped whit a 4 speed automatic 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 Silverado 1500 Check Engine Light With Code P1870
Chevrolet Silverado 1500 Problem
Drive Trains Affected: 4WD, Automatic Transmission
Average Mileage: 105,000 mi (50,000 mi - 160,000 mi)
Visitor, 2000 Chevrolet Silverado 1500, Automatic Transmission, 160,000 mi
With the a/c on and the cruise control engaged on a hot day I get the hard 1,2 shifts with the p1870 code.