Check Engine Light With Code P1870 on GMC 1500 Pickup

Problem Description and Possible Solution

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.

Problem Data
RepairPal Verified
Average mileage: 107,000 (107,000–107,000)
Drive Train affected: 2WD
4 model years affected: 1996, 1997, 1998, 1999
6 people reported this problem