Conditions for Setting the DTC
WARNING: This page is about a different car, the 2007 GMC Yukon XL, 2007 GMC Yukon, 2007 Chevrolet Tahoe, 2007 Chevrolet Suburban, and 2007 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
DTC P1818 sets when the TCM detects the following condition for 5 seconds.
PARK/NEUTRAL or FORWARD range indicated with a reverse gear ratio.