Normal Operation

The ABS module measures the amount of hydraulic brake pressure generated every time the brakes are applied. The ABS module also 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 regenerative 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 regenerative brake system recovers into brake-by-wire mode. If the regenerative 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 regenerative brake system downgrades to conventional brake pedal operation for the rest of the ignition cycle.

• DTC C0021:21 (Brake Booster Performance: Signal Amplitude □ Minimum) - If the brake booster is not able to build adequate hydraulic brake pressure when the brakes are applied either though input from the driver or when the ABS module checks the brake booster, this DTC sets. In most cases this DTC sets if the vehicle was started at low ambient temperatures, otherwise this DTC sets as a result of other brake booster DTCs or other DTCs that indicate a fault with ABS module inputs from the, brake pedal angle sensor, hydraulic brake pressure sensors or vacuum pressure sensor. 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. This DTC is for information only. 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.
• 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 downgrade to conventional mode until the cause of DTC C0021:21 has been corrected. The red brake warning indicator illuminates when this DTC is set.
• 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: 

• ABS module
• Brake booster
• Brake master cylinder
• HCU 
• Air in the hydraulic brake system
• Vacuum hoses, lines and fittings