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.
Check Engine Light With Code P1870 on GMC Savana 3500
Average mileage: 140,822 (82,000–199,645)
7 people reported this problem
2005 GMC Savana 3500 199,645 mi, Visitor
Transmission will down shift at any speed into 2 or 3rd gear with out warning.