Circuit/System Description
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The hybrid powertrain control module (HPCM) uses the GMLAN to signal the engine control module (ECM) that the hybrid system is requesting malfunction indicator lamp (MIL) illumination. This happens when the HPCM has determined that a failure affecting the hybrid system has occurred. When the ECM receives the message from the HPCM, DTC P0AC4 will set.