Diagnostic Aids

Make thorough inspection of wheel speed sensor circuitry. Pay particular attention to connectors, since connectors are the most likely area for problems.
Connect Tech 1 scan tester. Spin wheel by hand while monitoring wheel speed. Compare Tech 1 scan tester indications with those from other wheels.
Inspect wheel speed sensor for loose fastener or damage. Ensure gap between sensor and sensor ring is .025-.070" (.64-1.78 mm).
Suspected intermittents or opens at connectors can be detected with diagnostic service probe which permits voltage to be measured on wires without unplugging connectors. Ensure terminals make good connection.
When diagnosing intermittent problems, select MALF HISTORY from Tech 1 scan tester ABS menu. Supplemental data can be used to duplicate problem.

Fig 1: Code 21 - Schematic

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Fig 2: Code 21 - Diagnostic Flowchart (1 Of 2)

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Fig 3: Code 21 - Diagnostic Flowchart (2 Of 2)

Courtesy of GENERAL MOTORS CORP.