Engine Control Module (ECM)
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2020 GMC Yukon XL, 2020 GMC Yukon, 2020 Chevrolet Tahoe, 2020 Chevrolet Suburban, and 2020 Cadillac Escalade ESV. However, it is still accessible from the selected car via links, so may be relevant.
The ECM receives the BCM Prerelease Allowed signal and simultaneously the ECM sends a challenge to the BCM via the serial data circuit upon seeing the vehicle transition from OFF to any other power mode. Both the ECM and BCM perform a calculation on this challenge. If the calculated response from the BCM equals the calculation performed by the ECM, the ECM will allow vehicle starting.