Well $530 later, I had to replace the ignition wires and plugs back to OEM. According to the GM TSB. after market plugs and wires are subject to EMI which can throw these codes/alerts.
#PIC4536C: Service Charging System Message On DIC And DTC P0621 - keywords charge fuse generator horn lamp light open - (Dec 3, 2008)
￼Subject: Service Charging System Message on DIC and DTC P0621
Models: 2005-2009 Chevrolet Corvette models
2004-2009 XLR models
This PI was superseded to update model years. Please discard PIC4536B.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Some customers may comment that the Service Charging System message displays on the DIC. Using the Tech2, DTC P0621 is stored. This may be a result of aftermarket spark plugs or an open in circuit number 2540.
Check for aftermarket spark plugs. Plugs that are designed with a "Pulse" technology may interfere with this circuit and cause the Service Charging System message to illuminate on the IP and/or P0621 DTC store in the ECM. If aftermarket plugs are installed, replace plugs with OEM parts. This is not considered a warranty repair and all cost associated with this repair should be the customers responsibility.
Circuit 2540 is supplied voltage from the Horn fuse, position #2 in the UBEC. Apply the horn to see if it works. If the horn does not work, suspect an open fuse. This fuse may open if the horn is shorted due to water intrusion. If the horn is functional, check the integrity of circuit 2540 between the UBEC and the generator. Refer to SI document 1481125 for the schematic. Repair circuit 2540 as needed and re-evaluate the customers concern.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information. ￼ WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION