Normal Operation

WARNING: This page is about a different car, the 2010 Mercury Milan, 2010 Lincoln MKZ, and 2010 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.

The ABS module will measure the amount of hydraulic brake pressure generated every time the brakes are applied. The ABS module will also test 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 will utilize 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 will transition from brake-by-wire mode into low temperature conventional mode. While in low temperature conventional mode, the ABS module will continue to test the brake booster. If the brake booster begins to generate the expected amount of pressure in hydraulic system, the brake system will recover 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 will downgrade 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 will 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 will set. In most cases DTC C0021:21 will set if the vehicle was started in low ambient temperatures or as a result of low ambient temperatures causing slow booster reaction time. This DTC will also 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 will downgrade to conventional mode until the cause of DTC C0021:21 has been corrected. The red brake warning indicator will illuminate 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 will set. This DTC will 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