Check Engine Light With Code P1870 on GMC Savana 3500

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.

RepairPal Verified
Average mileage: 140,822 (82,000–199,645)
7 model years affected: 1996, 1997, 1998, 1999, more2000, 2005, 2006
7 people reported this problem
1 person shared problem details
2005 GMC Savana 3500199,645
Transmission will down shift at any speed into 2 or 3rd gear with out warning.
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