Code B127 - PRND321 Sensor: Notes

The PRND321 switch is a multi-signal switch sending information relative to gear selector position to BCM and ECM. Switch is mounted on transaxle assembly. The switch assembly also contains neutral safety switch, back-up light switch, and provides park input to automatic door lock module (if equipped). The switch uses 4 discrete circuits to pull BCM terminal voltages low in various combinations to indicate each gear selection. Voltage level of each circuit is represented in BCM data code BD41 as "0" for low, "1" for high. The 4 digits displayed represent decoder "A", "B", "C" and "PARITY" inputs in sequence.

The PRND321 switch also provides park/neutral input to the ECM in a similar manner using one discrete input. This can be seen using ECM input EI74 where "HI" or "LO" represent the voltage level of the input terminal.

Code B127 sets when BCM PRND321 input indicates park/neutral but ECM input does not, BCM PRND321 input does not indicate park/neutral but ECM input does, or decoder and parity inputs do not agree with possible combinations of "CORRECT VALUES" chart. If code B127 is set, CCDIC will display the "GEAR SELECT PROBLEM" warning and Instrument Panel Cluster (IPC) will flash "PRND321" display.

1. Checks BCM inputs when only decoder "A" and "PARITY" circuit are pulled low.
2. Checks BCM inputs only when decoder "B" and "C" inputs are pulled low. This will complete voltage check of all 4 BCM inputs.
3. Checks for a PRND321 switch problem in other gear ranges.
4. Checks if code is due to a discrepancy between the ECM and BCM inputs.