What should a technician do first when replacing a failed PCM?

When replacing a failed Powertrain Control Module (PCM), the first step is to disconnect the negative battery cable. This action is essential for several reasons. First, it ensures safety by preventing any accidental electrical shorts or shocks while working with the vehicle's electrical components. Disconnecting the battery also protects the new PCM from potential damage caused by residual voltage while connecting wires or making electrical adjustments.

By disconnecting the negative battery cable, the technician effectively resets the vehicle’s electrical system, allowing for a fresh start with the new module. After this step, the technician can proceed with the installation of the new PCM, and follow-up actions such as reprogramming and testing the system can be conducted in subsequent steps. This methodical approach is important to ensure that the PCM is installed correctly and operates as intended.