Conditions for Setting the DTC: P1826
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
The TCM detects IMS signal C is high, while the gear radio indicates first, second, third, or fourth gear for 8 seconds or greater.