Possible Causes

WARNING: This page is about a different car, the 2020 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

Incorrectly installed stoplamp switch
Brake drag (base brake system concern)
Wiring, terminals or connectors
HCU

AZ1: RECHECK FOR ABS MODULE DTC (DIAGNOSTIC TROUBLE CODE) C1A99

Ignition ON.
Using a diagnostic scan tool, clear ABS DTC s.
Test drive the vehicle at speeds above 20 km/h (12 mph) and perform at least 2 regular stops.
Ignition OFF.
Ignition ON.
Using a diagnostic scan tool, carry out the ABS self-test

Is DTC C1A99: xx currently present and active in the ABS module?

Yes	No
Go to AZ2.	GATHER information from the customer about conditions necessary to duplicate the concern. Some things to ask the customer may include; how often, drive time, down time, climate conditions (inside and outside the vehicle), speed traveling or maneuvering performed when the fault occurs. OPERATE the vehicle in the conditions outlined by the customers and MONITOR the PCM B001 and B002 PIDs, and the ABS module Hydraulic pressure PIDs to detemine a focus area. If there is pressure above 1,379 kPa (200 PSI) and no B00 reading, an incorrectly installed stoplamp switch is the most likely cause.

Yes

Go to AZ2.

GATHER information from the customer about conditions necessary to duplicate the concern.
Some things to ask the customer may include; how often, drive time, down time, climate conditions (inside and outside the vehicle), speed traveling or maneuvering performed when the fault occurs.
OPERATE the vehicle in the conditions outlined by the customers and MONITOR the PCM B001 and B002 PIDs, and the ABS module Hydraulic pressure PIDs to detemine a focus area.
If there is pressure above 1,379 kPa (200 PSI) and no B00 reading, an incorrectly installed stoplamp switch is the most likely cause.

AZ2: CHECK THE ABS (ANTI-LOCK BRAKE SYSTEM) MODULE HYDRAULIC PRESSURE PID (PARAMETER IDENTIFICATION)

NOTE: The brake pressure PID should read close to 0 kPa (0 PSI) with the brake pedal released and increase when applying the brake pedal.

Using a diagnostic scan tool, access the ABS and monitor the BRKHYDPRESS (Pressure) PID
While monitoring the PID, fully apply the brake pedal.

Does the PID read close to 0 kPa (0 PSI) without the brake pedal applied and increase when the pedal is pressed.

Yes	No
Go to AZ4	Go to AZ3.

AZ3: CHECK FOR BRAKE DRAG

With the transmission in NEUTRAL, position the vehicle on a hoist. Refer to DESCRIPTION AND OPERATION .
Spin each wheel by hand and check for any indication of brake drag.

Do all 4 wheels spin freely with little to no resistance?

Yes	No
Go to AZ4.	DIAGNOSE and REPAIR the brake drag concern. Refer to the appropriate Service Information.

AZ4: CHECK THE PCM (POWERTRAIN CONTROL MODULE) B001 and B002 PID (PARAMETER IDENTIFICATION)

Ignition ON.
Monitor the following PID while pressing and releasing the brake pedal.
Access the PCM PID and monitor the B001 (Status) PID
Access the PCM and monitor the B002 (Status) PID

Do the PIDs indicate ON with the brake pedal applied and OFF with the brake pedal released?

Yes	No
CHECK OASIS for any service articles: TSB, GSB, SSM or FSA. If a service article exist for this concern, DISCONTINUE this test and FOLLOW the service article instructions. If no service articles address the concern, INSTALL a new HCU. Refer to HYDRAULIC CONTROL UNIT (HCU) .	INSTALL a new Brake ON/OFF switch. Refer to STOPLAMP SWITCH .