P1870 is typically relating to TCC slippage. This can only be verified with a scan tool and watching data while going down a long road/highway in lockup. The
pcm wants to see a steady slippage between 2 and 50 rpm and if this is not maintaned it will throw the code after a while. This is often caused by a worn tcc valve in the valve body and there are a number of ways to fix it. The Transgo shift kits, regular or performance, will come with an upgraded valve for the valve body that makes lockup a full time on or off setup so there is no slippage and it will feel more like shifting into another gear. There are other companies such as Sonnax that sell new TCC valves for the valve body to restore new operation but require a special reamer to install the valve. This is of course if the valve is your problem but again is pretty common in the 4L60E and since it is 100% electronically controlled it must be properly diagnosed with a scan tool to verify exactly what is going on.
Linear Mode
