Possible Causes

WARNING: This page is about a different car, the 2019 Ford F-550 Super Duty, 2019 Ford F-450 Super Duty, 2019 Ford F-350 Super Duty, and 2019 Ford F-250 Super Duty. 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

CK1: RECHECK FOR ABS MODULE DTC C1A99

Ignition ON, engine OFF.
Carry out the ABS self-test.
Is DTC C1A99:xx currently present and active in the ABS module?

Yes	No
Go to CK2.	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 customer and MONITOR the PCM BOO1 and BOO2 PIDs, and the ABS module Hydraulic pressure PIDs to determine a focus area. If there is pressure above 1, 379 kPa (200 PSI) and no BOO reading, an incorrectly installed stoplamp switch is the most likely cause.

Yes

Go to CK2.

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 customer and MONITOR the PCM BOO1 and BOO2 PIDs, and the ABS module Hydraulic pressure PIDs to determine a focus area.
If there is pressure above 1, 379 kPa (200 PSI) and no BOO reading, an incorrectly installed stoplamp switch is the most likely cause.

CK2: CHECK THE ABS MODULE HYDRAULIC PRESSURE PID

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.

Access the ABS and monitor the BFLHPC (PRESS) 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 CK4.	Go to CK3.

CK3: CHECK FOR BRAKE DRAG

With the transmission in NEUTRAL, position the vehicle on a hoist. Refer to the Jacking and Lifting article.
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 CK4.	DIAGNOSE and REPAIR the brake drag concern. REFER to Brake System General Information article.

CK4: CHECK THE PCM BOO1 AND BOO2 PIDS

Ignition ON, engine OFF.
Monitor the following PIDs while pressing and releasing the brake pedal.
Access the PCM and monitor the BOO1 (MODE) PID.
Access the PCM and monitor the BOO2 (MODE) PID.
Do the PIDs indicate ON with the brake pedal applied and OFF with the brake pedal released?

Yes	No
INSTALL a new HCU. CHECK OASIS for any service articles: TSB, GSB, SSM or FSA. If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions. If no service articles address this concern, INSTALL a new ABS module and HCU assembly. REFER to the Anti-Lock Brake System (ABS) and Stability Control article. ORDER a replacement ABS module and HCU assembly. INCLUDE the RVC on the form. If this is a Ford paid repair, INCLUDE the RVC on the warranty claim form.	INSTALL a new BOO switch. REFER to the Exterior Lighting article. When installing the new switch, USE the appropriate flex probe to VERIFY correct pin fitment at the connector.

Yes

INSTALL a new HCU.
CHECK OASIS for any service articles: TSB, GSB, SSM or FSA.
If a service article exists for this concern, DISCONTINUE this test and FOLLOW the service article instructions.
If no service articles address this concern, INSTALL a new ABS module and HCU assembly. REFER to the Anti-Lock Brake System (ABS) and Stability Control article.
ORDER a replacement ABS module and HCU assembly. INCLUDE the RVC on the form.
If this is a Ford paid repair, INCLUDE the RVC on the warranty claim form.

INSTALL a new BOO switch.
REFER to the Exterior Lighting article. When installing the new switch, USE the appropriate flex probe to VERIFY correct pin fitment at the connector.