A45 With AC9

DTC B2121 Head Restraint Up/Down Sensor Failed (A45 with AC9):

DTC B2121 Head Restraint Up/Down Sensor Failed (A45 with AC9) (Part 1 of 2):

DTC B2121 Head Restraint Up/Down Sensor Failed (A45 with AC9) (Part 2 of 2):

CIRCUIT DESCRIPTION
The Memory Seat Module (MSM) receives inputs from the headrest position sensor during memory recall and memory set functions. The potentiometer in the position sensor varies in resistance as the motor moves the headrest up and down. The MSM receives a variable voltage input depending on the resistance of the potentiometer. During memory set, the MSM records the voltage across the headrest potentiometer. During memory recall, the MSM moves the headrest motor until the voltage across the potentiometer matches the voltages stored in memory. The headrest is not moved during the easy exit function. During normal operation, the headrest position sensor is checked against out-of-range voltage limits.

CONDITIONS FOR SETTING THE DTC
^ The MSM will set the DTC B2121 any time it detects that the headrest sensor input voltage is out of range (less than 0.25 volts or greater than 4.75 volts).
^ DTC B2121 will set only if DTC B1982 MSM or DTC B1983 MSM is not current.

ACTION TAKEN WHEN THE DTC SETS
When DTC B2121 is set as current, the Memory Seat Module (MSM) will disable the headrest motor during memory recall operation. The headrest motor will remain enabled for manual positioning.

CONDITIONS FOR CLEARING THE DTC
^ The MSM detects that the headrest position sensor input is within the valid voltage range.
^ Use the On-Boards clearing DTCs feature.
^ Use a scan tool.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
1. Perform the Power Seat Systems Diagnostic System Check before continuing with the diagnosis of this DTC.
2. Multiple position sensor DTCs set indicates a concern with the reference wiring or memory seat module (MSM). Only one position sensor DTC set indicates a concern with the position sensor and wiring.
21. Clear all DTCs after the repair procedure is complete.