Important
DTC P1870 is a type B code. The conditions for setting the P1870 DTC must occur on TWO CONSECUTIVE TRIPS (ignition cycles, with a drive cycle) before setting a P1870 history code.
When the conditions for setting DTC P1870 are met (first trip), the PCM commands maximum line pressure and harsh 1-2 shifts are the result.
This may result in a harsh 1-2 shift with no history code if the conditions for setting the DTC required for the second trip are not met, on two consecutive trips (Ignition cycles, with a drive cycle).

When the conditions for setting the DTC are met, on the second consecutive trip, a DTC P1870 is stored as a history code.
When the P1870 code is stored, the PCM will turn on the Service Engine Soon (SES), Check Engine Light (CEL), or Malfunction Indicator Lamp (MIL).
Correction
Install a control valve body with the revised TCC regulator and isolator valves. These valves are used in all transmissions produced after January 15, 1999 (Julian Date 9015), and all of the service parts currently available through GMSPO contain revised TCC regulator and isolator valves.

Important
If all of the following conditions are true, it is not necessary to rebuild the transmission or to replace additional transmission components beyond the control valve body.

My thoughts are wreaking yard and car after 1999. I found this information for my 99 when I had a 1875 code. And shift problems. It thought it was in 4 wheel low and it was not. Problem for me is not resolved, but is not a problem at this time. I found that I could remove the ATC fuse ( automatic transfer case) and that disconnect all that had to do with 4 wheel drive. Shift problems are gone. Ordered new switch, came yesterday but did not repair the 4 wheel drive problem. I live in Hawaii and really don't need the 4 wheel at this time. I think my next step will be to scan the system. This car has certainly provided a lot of entertainment for me. I always have something that needs fixing.

 

I chickened out on vacation and didn't mess with installing the upgrade TCC valve. Instead, I added a pint of Seafoam transmission treatment, drove it to work all week and didn't get a CEL-not yet anyway since it still shifts hard between 1st and 2nd. I don't know-maybe the treatment is helping the valve shift a little easier and not so much that it triggers a code. I siphoned out the pint of transmission fluid giving me the idea to siphon out the entire pan. It would be a huge plus not having all the fluid run out when I pull the pan. I'm halfway tempted to try doing it this afternoon. Just not sure if that'll be enough time to do all of it. I've watched the video where they were able to remove the retaining pin on the valve spool by bending the ears in, and spreading them with a narrow set of needle nose that allows the stock valve to come out without removing the entire valve assembly.
Reading up on the p1870 code it's just a general 'slip' error the diagnostic computer is sensing. It would really suck if that TCC valve isn't the problem, but 90% of the time it is. I understand a hard shift between 1st and 2nd gear is the computer compensating for slip it's sensing by sending max hydraulic pressure. I'm just a little worried over taking apart a running vehicle and goofing up something and turning it into an expensive parking lot weight! From the directions I gather you remove everything except the very tip of the old valve spool and leave out the plug as the upgrade is solid with one. (And an extra set of o-rings a smaller diameter if the bore isn't too wallowed out.) Others sites I've read recommend actually reaming out the bore-but I've watched more than one video where they were able to install the upgrade without doing this. Long story short-think I can possibly do this in an afternoon?