Theory Of Operation

This diagnostic checks for the validity and availability of the Shift Lever Position signal which is sent by the Electronic Shift Module (ESM) via a CAN message. A CAN signal state is defined for each of five stable lever positions: 'Park', 'Reverse', 'Neutral', 'Drive' and 'Autostick'. CAN signal states are defined for six non-stable positions: 'Up', 'Down', 'P-R', 'R-N', 'N-D' and 'D-A'. The diagnostic checks the validity by verifying the received CAN signal state corresponds to one of the possible lever positions. The diagnostic also checks the availability by verifying the valid bit. When the Shift Lever Position signal is invalid or missing, then DTC U1420 will set.