Conditions for Setting the DTC: P1820
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 IMS signal A is continuously low this ignition cycle, and the IMS indicates transitional position D4-D3 for 8 seconds or greater.