A021

Fig. 275 Code A021: Left Front Wheel Speed = 0 Or Unreasonable (Part 1 Of 3):

Fig. 275 Code A021: Left Front Wheel Speed = 0 Or Unreasonable (Part 2 Of 3):

Fig. 275 Code A021: Left Front Wheel Speed = 0 Or Unreasonable (Part 3 Of 3):

DTC A021 Left Front Wheel Speed = 0 Or Unreasonable:

Circuit Description:

As a toothed ring passes by the wheel speed sensor, changes in the electromagnetic field cause the wheel speed sensor to produce a sinusoidal (AC) voltage signal whose frequency is proportional to wheel speed. The magnitude of this signal is directly related to wheel speed and the proximity of the wheel speed sensor to the toothed ring (often referred to as the air gap).


Failure Condition:

DTC AO21 can be set when the vehicle is not in an ABS stop. If the left front wheel speed is less than 1/2 of the vehicle's reference speed and the vehicle's reference speed is greater than 8 km/h (5 mph), a malfunction exists.


Action Taken:

A DTC AO21 is stored, ABS is disabled and the "ABS" indicator is turned on.


Test Description:

Number(s) below refer to circled Number(s) on the diagnostic chart.
1. This test verifies whether the malfunction is currently present.
2. This step will identify a wheel speed sensor or circuitry that is damaged and visibly apparent.
3. This test checks wheel speed sensor for proper resistance values.
4. This test ensures the wheel speed sensor and sensor ring generate the proper voltage.
6. This test checks for an open in CKT 9KO1.
7. This test checks for an open in CKT 9L01.
8. This test checks for short to ground in CKT 9KO1 or CKT 9L01.
9. This checks for a short to voltage in CKT 9L01 or CKT 9K01.
10. This checks for shorted wheel speed signal circuits.
11. This step ensures that DTC A021 was not set due to a poor connection between the EBCM and connector C1.


Diagnostic Aids:

An "intermittent" malfunction may be caused by a poor connection, rubbed through wire insulation or a wire that is broken inside the insulation. The frequency of the malfunction can be checked by using the enhanced diagnostic function of the TECH 1.



If the customer's comments reflect that the "ABS" indicator is on only during moist environmental changes (rain, snow, vehicle wash, etc.), all wheel speed sensor circuitry should be thoroughly inspected for signs of water intrusion. Use the following procedure:

1. Spray down the suspected area with 5% salt water solution (10 ml [2 teaspoons] of salt to 355 ml [12 fl. oz.] of water).

2. Start vehicle and allow it to run for ten seconds.

3. If the DTC returns immediately, replace the suspected harness.

Any circuitry that is suspected of causing the intermittent complaint should be thoroughly checked for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wiring connections or physical damage to the wiring harness. When replacing a wheel speed sensor, inspect the sensor terminals and harness connector for corrosion and/or water intrusion. If evidence of corrosion or water intrusion exists, replace the wheel speed sensor.