Normal Operation

WARNING: This page is about a different car, the 2011 Mercury Milan, 2011 Lincoln MKZ, and 2011 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.

The ABS module measures the amount of hydraulic brake pressure generated every time the brakes are applied. The ABS module tests the brake booster every key cycle by operating the brake booster independently of brake pedal input and measuring the hydraulic brake pressure generated. The ABS module utilizes input from the brake pedal angle sensor, brake booster travel sensor, the brake pressure sensors internal to the Hydraulic Control Unit (HCU) and the vacuum pressure sensor to determine if the required amount of hydraulic brake pressure is being generated. If the ABS module detects that the brake booster is not able to build adequate hydraulic brake pressure in the master cylinder, the brake system transitions from brake-by-wire mode into low temperature conventional mode. While in low temperature conventional mode, the ABS module continues to test the brake booster. If the brake booster begins to generate the expected amount of pressure in hydraulic system, the brake system recovers into brake-by-wire mode. If the brake system has to transition from brake-by-wire mode to low temperature conventional mode 3 times in the same ignition cycle because of a brake booster performance fault, the brake system downgrades to conventional brake pedal operation for the rest of the ignition cycle.

DTC C0021:21 (Brake Booster Performance: Signal Amplitude □ Minimum) - This DTC is for information only and does not cause any indicators to illuminate. However, other DTCs that cause DTC C0021:21 to set may cause indicators to illuminate. If the red brake warning indicator is illuminated, diagnose all other DTCs before diagnosing DTC C0021:21. Otherwise, disregard DTC C0021:21.
- If the brake booster is not able to build adequate hydraulic brake pressure when the brakes are applied either through input from the driver or when the ABS module checks the brake booster, DTC C0021:21 sets. In most cases DTC C0021:21 sets if the vehicle was started in low ambient temperatures or as a result of low ambient temperatures causing slow booster reaction time. This DTC set as a result of other brake booster, brake pedal angle sensor, hydraulic brake pressure sensors or vacuum pressure sensor DTCs being set.
DTC C0021:66 (Brake Booster Performance: Signal Has Too Many Transitions) - This DTC indicates that the regenerative brake system has switched from brake-by-wire mode to low temperature conventional mode 3 times due to the cause of DTC C0021:21. When this DTC is present, the regenerative brake system downgrades to conventional mode until the cause of DTC C0021:21 has been corrected. The red brake warning indicator illuminates when this DTC is set. If only DTCs C0021:21 and 66 are present, there is likely to be mechanical damage to the brake booster, brake pedal and bracket, master cylinder, a hydraulic brake fluid leak, air trapped in the hydraulic system or a vacuum leak may be present.
DTC C0021:77 (Brake Booster Performance: Commanded Position is Not Reachable) - This DTC is for information only and does not indicate a fault with the brake system. If the hydraulic brake pressure does not change when the ABS module checks the brake booster, this DTC sets. This DTC does not cause any indicators to illuminate and may set as a result of low ambient temperatures causing slow booster reaction time. If the red brake warning indicator is not illuminated, this DTC should be disregarded. Diagnose all other DTCs retrieved along with this DTC.

This pinpoint test is intended to diagnose the following:

Wiring, terminals or connectors
Wheel speed sensor
ABS module