Possible causes include:
* Faulty Transmission Range Sensor (TRS): This sensor tells the PCM what gear the transmission is in. A malfunctioning TRS can cause erratic shifting or the inability to shift.
* Problems with the Transmission Shift Solenoids: These electrically controlled valves direct hydraulic pressure to engage different gears. A malfunctioning solenoid will prevent the transmission from shifting correctly.
* Wiring Issues: Damaged, corroded, or loose wiring in the transmission harness can interrupt the electrical signals to and from the transmission.
* PCM Malfunction: The PCM interprets the signals from various sensors and controls the solenoids. A faulty PCM can cause a wide range of transmission problems.
* Low Transmission Fluid: While not directly an electrical problem, low fluid can cause sensor readings to be inaccurate, triggering the malfunction code.
* Faulty Speed Sensors: Input shaft speed sensor (ISS) or output shaft speed sensor (OSS) malfunctions will affect the PCM's ability to control the transmission shifts.
What to do:
1. Get the diagnostic trouble codes (DTCs): A scan tool (OBD-II reader) is necessary to get the specific codes. This will pinpoint the likely source of the problem. Auto parts stores often offer free code reading services.
2. Check the Transmission Fluid: Ensure the fluid level is correct and the fluid itself is clean and doesn't smell burnt.
3. Inspect Wiring: Visually inspect the wiring harness leading to the transmission for any obvious damage, corrosion, or loose connections.
4. Professional Diagnosis: Because of the complexity of the transmission control system, it's highly recommended to take the Tahoe to a qualified mechanic or transmission specialist for a proper diagnosis and repair. Attempting DIY repairs without proper knowledge can lead to more extensive damage. The mechanic will use a scan tool, test the sensors and solenoids, and potentially perform a fluid change to address the problem.
