Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1249 (CELLULAR COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESONSTAR REPROGRAMMINGVEHICLE COMMUNICATION UNIT (VCU)
2003 Chevrolet TrailBlazer 4.2 S, 4WD
Vehicle Communication Unit (VCU)
2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Vehicle Communication Unit (VCU)
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
- Record the 11 digit Electronic Serial Number (ESN) number from the label on the new module.
- Install the new VCU.
- After replacement, press and hold the white "Dot" button until the OnStar status LED turns from green, to red, and then back to green.
- Install a scan tool. Use the I.D. Information menu to access the new ESN.
- Move the vehicle to an open area that is away from tall buildings, and with a clear view of unobstructed sky. Allow the vehicle to run for 10 minutes.
- Press the blue OnStar button to connect to the OnStar Call Center. Tell the advisor that this vehicle has received a new VCU and ask them to perform the following:
- Add the new ESN to update the customer's account.
- Activate the OnStar Personal Calling feature if it is available in the area.
- Perform a full reconfiguration on the vehicle.
- The OnStar® Advisor will reconfigure the vehicle with you. Once the advisor completes the reconfiguration, you can end the call. Follow any additional instructions given by the Advisor.
- The old VCU must be returned for core credit with the address label included in the original box. Failure to return core will result in a debit.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.