Diagnostic Aids

Communication DTCs and/or concerns need to be addressed first.
Only Replace a Component if the DTC resets during a drive the current drive cycle
If any of the DTCs P3051 thorough P3056 are set inspect the T19 Power Supply Control Module connector for evidence of Corrosion or loose terminals and replace as necessary, if no terminal issues are found. apply a light coat of Dielectric grease to the terminals, clear DTC's and revaluate after performing 5 stop start events.
If both DTC P3055 & DTC P3056 are set, the condition may be caused by an internally shorted starter, starter wiring circuits may be shorted together, or a lack of voltage on the T19 Power Supply Control Module Ignition Volage Terminal 6.
If DTC P3055 is set, but DTC P3056 is not set the condition may be caused by an over-current event on the T19 Power Supply Control Module Regulated voltage output 1, that can cause the module to stop regulating voltage.
The K20 ECM is responsible for setting the voltage codes, since the DC/DC converter cannot set DTC's. The K20 ECM monitors the DC/DC Converter voltages via the data line. If the K20 ECM loses communication with the DC/DC Converter for any reason the K20 ECM will not be able to monitor the voltages, therefore the K20 ECM interprets this as a loss of voltage and DTCs P3051 and/or P3055 will set as a result.
If DTC P3056 is set, verify K20 ECM and T19 Power Supply Transformer grounds are clean and tight.
Ground circuits for the area of customer concern are clean, tight, and in the correct location.
Verify all devices powered by the T19 Power Supply Transformer are operating properly.
B+ circuit terminal 8 supplies voltage to output circuit terminal 9, function 12 V regulated supply voltage 1.