VSC light comes on with error code PO603 for Lexus GS-300

Question

Error cod PO603 dealer is recommending engine carbon build up cleaning - $10,000

Answer 1

P0603 Internal control module (keep alive memory)

Anytime the CK ENG LT comes on the VSC is turned off this is due to the vehicle not operating at  manufacturer  specification. As for P0603 this is for the ECM