What Does C1939 Mean?
The ABS/stability control module has detected an electrical circuit fault in the brake pressure switch input, meaning it cannot determine whether the driver is applying the brake pedal. This switch provides a critical input for ABS, traction control, brake-assist, and shift-lock release functions. The driver may notice the ABS or brake warning light illuminated, and features like brake assist or hill-hold may not function.
Common Causes
35%
Faulty brake pressure switch (brake light switch) with open or intermittent circuit
25%
Damaged or corroded wiring between the brake switch and the ABS module
20%
Brake switch out of adjustment — not making contact at the pedal
10%
Blown fuse in the brake switch circuit
10%
ABS/ESC module internal circuit fault on the brake switch input pin
Diagnostic Steps
1
Check if the brake lights illuminate when the pedal is pressed. If they do not, the brake switch or its circuit is likely at fault. Check the brake light fuse first.
2
Access the brake pressure switch at the brake pedal bracket. With the ignition on, backprobe the switch connector and verify you get battery voltage on the supply wire. Press the pedal and confirm the signal wire toggles between 0V and battery voltage.
3
If the switch has proper voltage but the module still reports a fault, check for continuity and resistance on the wiring from the switch connector to the ABS module connector. Look for chafed wires near the steering column and firewall pass-through.
4
Inspect the brake switch adjustment — the switch plunger should depress when the pedal is released and extend when the pedal is pressed. Verify the retaining clip and pedal bumper are intact.
5
Monitor the brake switch PID in live data while pressing and releasing the pedal. The module should show ON/OFF toggling. If it is stuck in one state, the fault is confirmed.
Estimated Repair Cost
$20 - $200
Parts + labor, varies by vehicle and location
The ABS/stability control module has detected an electrical circuit fault in the brake pressure switch input, meaning it cannot determine whether the driver is applying the brake pedal. This switch provides a critical input for ABS, traction control, brake-assist, and shift-lock release functions. T...
The most common cause of C1939 (Brake Pressure Switch Input Circuit Failure) is: Faulty brake pressure switch (brake light switch) with open or intermittent circuit
Typical repair costs for C1939 range from $20 to $200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Basic braking function is not affected, but ABS, brake assist, traction control, and cruise control cancellation may not work properly. The brake lights may also be inoperative, creating a serious rear-end collision risk. Repair promptly — if brake lights are not working, do not drive the vehicle.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to C1939 to identify the root cause.
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Quick Info
Category
Chassis
System
ABS / Brake System
Difficulty
Type
Manufacturer
Recommended Tools
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