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 Sonoma Check Engine Light With Code P1870
GMC Sonoma Problem
Drive Trains Affected: 2WD, 4WD, Automatic Transmission
Average Mileage: 148,110 mi (111,000 mi - 201,662 mi)
Visitor, 1999 GMC Sonoma, 2WD, 171,000 mi
Check engine light came on and had it checked. Code P1870 shows up. Noticed right after it came on that transmission was shifting hard going from 1st to 2nd. Trans. works fine until engine warms up and I have been stop and go city driving.I had the code reset but it came back on after 190 miles then the shift from 1st to 2nd. was harder.
Visitor, 1998 GMC Sonoma, 2WD, 120,000 mi
Check engine light on with p1870 code, (from advance auto).