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
GMC Sierra 1500 Check Engine Light With Code P1870
GMC Sierra 1500 Problem
Drive Trains Affected: 2WD, 4WD, Automatic Transmission
Average Mileage: 121,000 mi (18,000 mi - 180,000 mi)
Rough shift after N-O-T is reached in 1st and 2nd gear.Also it seems to want to slip out of 4th gear and fluctuate RPM's by 200 or 300 while throttle is constant.On flat ground only.,not on hills though!
Visitor, 1999 GMC Sierra 1500, 2WD, 180,000 mi
hard shift from 1st to 2nd. Sometimes 2nd to 3rd. Very erratic. Comes and goes without notice. Check engine light will come on with a transmission code. Goes back off after a couple of days normal driving.
Visitor, 1999 GMC Sierra 1500, Automatic Transmission, 18,000 mi
hot summer weather after 50 miles, hard shift from 1st to 2nd, only does it on hot summer days. Not fixed