Spicer TPCS Troubleshooting Guide - Manual (Page 37)

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Service Codes
Group: 3P! (Code 21)
Vacuum Fault (Code 21)
Replace or clean PCU.
Caution: Verify clean (no oil), dry air system on vehicle. This
condition can be caused by contamination in PCU. Reference
PCU Cleaning and Inspection for cleaning procedure.
Verify the repair.
Verify the problem.