I'm checking the sites to find out what code P1870-C stands for. My list doesn't have this code in it.

 

Got this from LS1edit.com

Info - Diagnostic Information on DTC P1870 #02-07-30-001
Diagnostic Information on DTC P1870
2000-2002 Passenger Cars and Light Duty Trucks

with a 4L60E or 4L65E Automatic Transmission

Diagnostic trouble code (DTC) P1870, Transmission Component Slipping, may
have different root causes. The following diagnostic tips are provided to
assist in accurately repairing the condition.

Corporate Bulletin Number 01-07-30-023A, dated November 2001, is intended
for 1996-1999 vehicles with more than 32,000 km (20,000 mi). This bulletin
addresses wear in the valve body TCC isolator/regulator bore which can
cause DTC P1870. This wear condition is not the cause of DTC P1870 on any
2000 model year vehicle built after January 15, 2000, or any 2001 or 2002
vehicles as the valve body used on these vehicles has been revised to
prevent the wear condition.
Corporate Bulletin Number 99-07-30-005, dated March 1999, should be used
to help with diagnosis of this DTC. This bulletin will help to determine
if the DTC is caused by the TCC system or by a slipping internal
transmission component such as a clutch or band.
A plugged or restricted TCC apply fluid orifice (238, located in the pump)
may cause DTC P1870, especially in low mileage vehicles with less than
8,000 km (5,000 mi).
A converter clutch valve (224, located in the pump) which does not have
full travel of 13 mm (0.5 in) or is stuck in the off position may cause
DTC P1870. Inspect the valve for chips, debris, nicks or burrs. Also,
inspect the springs (225 and 226) to make sure they are not limiting valve
travel. A spring that is not fully wound may catch in the bore, limit
valve travel and cause DTC P1870.
A cracked or leaking TCC solenoid (part of harness 66) may cause DTC
P1870. Use solenoid test kit J 44246 to test the operation of this
solenoid.
A cracked or leaking TCC PWM solenoid (396) may also cause DTC P1870.
When attempting to diagnose a DTC P1870, it is important to also refer to
the appropriate Service Manual for further possible causes of this
condition.

 

The manual for 2001 vehicles states the following:
======================================
The Powertrain Control Module (PCM) monitors the difference between engine speed and transmission output speed. In D3 with the Torque Converter Clutch (TCC) engaged, the engine speed should closely match the transmission output speed. In D4, with the TCC engaged, the TCC slip speed should be -20 to +50 RPM.

When the PCM detects excessive TCC slip when the TCC should be engaged, then DTC P1870 sets.
======================================
The very first diagnostic step is to check the transmission fluid level - let's hope that fixes it !! :thumbs:

 

Although there are many different reasons this code cane appear here is another TSB.

Condition
Some customers may comment on a harsh 1-2 upshift and the Check Engine Light or Service Engine Soon indicator is illuminated.

Diagnosis


Typically, these vehicles will have been driven more than 32,000 km (20,000 mi) before this condition occurs.

The scan tool may show a DTC P1870 set as a history code.

A harsh 1-2 shift or DTC P1870, caused by wear in the control valve body, may be difficult to duplicate when the transmission temperature is below 93°C (200°F).

Cause
The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply.


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. Refer to the Parts Information table below. These valves are used in all transmissions produced after January 15, 2000 (Julian Date 0015), 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.

Transmission operation is normal before the transmission reaches operating temperature, or before DTC P1870 is set (no slips, flares, or missing gears).
The torque converter is not blue or overheated.
The transmission fluid is not burned or has no burned odor.
The transmission fluid pan contains no abnormal debris (clutch material, bronze, brass, or metal fragments).