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.
Average mileage: 205,590 (169,000–268,363)
Drive Train affected: 4WD
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Had same problem on my high mileage (245k)Tahoe. My transmission technician said code referred to worn valve body and solenoid that regulated hydraulic pressure. $1500 for overhaul/ replacement or add a spring to hold solenoid open applying full pressure instead of partial pressure. Result, no codes, smooth shifting and cost was $250 instead of $1500 !The guy is a genius.
I have a 2002 chevy tahoe that Throws code p1870
My car is throwing this code, and I've just had my transmission rebuilt. Back to the shop it goes, I guess.