A microcomputer controls the transmission by receiving and processing signals from various switches and sensors. The microcomputer determines shift sequences, shift timing, and clutch apply and release characteristics. The microcomputer is an independent controller and is referred to as a transmission control module (TCM). The pressure switch manifold (PSM) and the park/neutral position (PNP) switch provide operator input to the TCM. Other data sent to the TCM include throttle position, engine, turbine, and output speeds, and sump temperature. Any active special function, such as anti-lock brakes or power take-off, is also an input to the TCM. The TCM processes this data to determine proper shift points, to monitor the current range, to perform ratio tests, and to compile diagnostic data. The TCM is programmed to protect the transmission and other vehicle driveline components by inhibiting actions, such as full-throttle NEUTRAL-to-range shifts and high-speed direction changes. The TCM determines if a system malfunction exists and stores diagnostic codes related to the malfunction. The codes accessed by the service technician are used in diagnosing persistent or intermittent trouble in the system.

To obtain transmission diagnostic trouble codes (DTCs), install the scan tool and follow the menu selections listed below:

1. From the title screen, press ENTER.
2. Select F0: Diagnostics. Press ENTER.
3. Select Model Year. Press ENTER.
4. Select LDTrk, MPV, Incomplete. Press ENTER.
5. Select Product Make, i.e. (C) Chevrolet Truck, (T) GMC Truck. Press ENTER.
6. Select Product Line, i.e. (C) or (K). Press ENTER.
7. Select F0: Powertrain. Press ENTER.
8. Select Engine size. Press ENTER.
9. Select 6 Speed Automatic. Press ENTER.
10. Select F0: Diagnostic Trouble Codes (DTC). Press ENTER.
11. Select F1: DTC (s) (Transmission). Press ENTER.
12. Select F0: DTC Information. Press ENTER.
13. Select F0: DTC Info. Press ENTER.

Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data.