This GM manufacturer-specific code indicates an issue with the transmission pressure/solenoid system. Code P1821 requires GM-specific diagnostic procedures and potentially dealer-level scan equipment.
Find certified mechanics near you who can diagnose and repair P1821.
For informational purposes only. Diagnostic trouble codes are starting points for diagnosis, not definitive answers. Always consult a certified mechanic for proper diagnosis and repair. Estimated costs vary by vehicle, location, and shop rates.