Possible Causes

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

CT1: 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 CT2.	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 CT2.

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.

CT2: 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 BRKHYDPRESS (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 CT4.	Go to CT3.

CT3: CHECK FOR BRAKE DRAG

With the transmission in NEUTRAL, position the vehicle on a hoist. Refer to Jacking and Lifting.
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 CT4.	DIAGNOSE and REPAIR the brake drag concern. REFER to the Brake System General Information article.

CT4: 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?