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 Yukon XL 2500 Check Engine Light With Code P1870
GMC Yukon XL 2500 Problem
Model Year Affected: 2000
Visitor, 2000 GMC Yukon XL 2500, 50,000 mi
The problem began at around 30K miles with a chatter in the transmission chatter. At around 50K we started having the hard shifts and Check Engine Lite. We now have around 110K and it still shifts hard after driving on the Freeway. When it happens, I either accelerate to the shift point then let up on the throttle to allow a low power shift which is easier on the drive line, and my neck, or if at a stop light I put it in Park, shut down the engine for a few seconds, then restart and drive away on the green. This allows the computer to reset the transmission to allow it to operate normally until the next trip on the freeway.
Apparently the problem is caused by a leaky TCC solenoid which does not allow the torque converter to lock up completely allowing slippage within the converter. When the computer detects a 300(I believe)RPM difference between the input and output of the transmission it increases the fluid pressure in the transmission to lock the converter, this however causes rapid, hard shifts.
Replacing the solenoid in the Valve Body is suppose to be an easy fix if your handy, I haven't built up the courage to try it, or the $5,000 to replace the transmission if I screw it up. I've just taught myself how to live with it. I hate GM, every one I've owned has been a nightmare.