P2120, P2122, P2123, P2125, P2127, P2128

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2011 GMC Yukon XL, 2011 GMC Yukon, 2011 Chevrolet Tahoe, 2011 Chevrolet Suburban, and 2011 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0641 or P0651 is not set.
The ignition is ON or the engine is running.
The run/crank or powertrain relay voltage is greater than 6.0 V and reduced power is not active.
DTC P2120, P2122, P2123, P2125, P2127, P2128 run continuously when the above conditions are met.