C1939

What Does Code C1939 Mean?

The diagnostic trouble code C1939 signifies a detected malfunction within the input circuit of the brake pressure switch. This code is typically set by the Anti-lock Braking System (ABS) module, Traction Control System (TCS), or Electronic Stability Control (ESC) module, which are often integrated and collectively responsible for monitoring critical brake system inputs. The brake pressure switch is a vital component that measures the hydraulic pressure within the brake lines, converting this physical input into an electrical signal. This signal is crucial for various vehicle dynamic control systems, providing information about brake application force. When the control module detects an electrical reading from this circuit that falls outside its calibrated operational parameters—such as an abnormally high voltage (indicating an open circuit or short to voltage), an abnormally low voltage (indicating a short to ground or open circuit), or an implausible signal that does not correlate with other sensor inputs (e.g., brake pedal position sensor)—it interprets this as a “circuit failure” and sets the C1939 code. The primary subsystem affected is the vehicle’s active safety braking and stability control systems, as they rely on accurate brake pressure data for proper operation.

Common Symptoms

• Illumination of the ABS warning light on the instrument cluster.
• Illumination of the TCS and/or ESC warning lights.
• The cruise control system may become inoperative.
• Potential deactivation or malfunction of brake-related driver assistance systems (e.g., hill-start assist, automatic emergency braking).
• In some vehicle architectures, a slightly altered transmission shifting characteristic or torque converter lock-up behavior may occur due to inhibited brake signal input to the Powertrain Control Module (PCM).
• A slightly firmer or spongy brake pedal feel if the brake assist system relies on this input for activation, though this is less common directly from a circuit failure.

What Causes the Code C1939?

• Defective Brake Pressure Switch: The most common cause, involving internal electrical failure, sensor drift, or an inability to accurately generate or transmit a signal.
• Wiring Harness Damage:
  • Open Circuit: A broken or disconnected wire in the signal, reference voltage, or ground circuit.
  • Short Circuit to Ground: The signal wire or reference voltage wire making unintended contact with a chassis ground.
  • Short Circuit to Voltage: The signal wire or ground wire making unintended contact with a positive voltage source.
• Corrosion or Loose Connections: Degraded terminals within the brake pressure switch connector or at the ABS/TCS/ESC module connector leading to high resistance or intermittent signal loss.
• Contamination: Ingress of moisture, brake fluid, or other contaminants into the switch connector causing electrical bridging or corrosion.
• Faulty ABS/TCS/ESC Control Module: While less frequent, an internal failure within the control module’s input circuit can incorrectly interpret or fail to read the brake pressure switch signal.

How to Diagnose and Troubleshoot

A systematic diagnostic approach is essential to accurately pinpoint the cause of C1939:

1. Visual Inspection:
   • Begin by locating the brake pressure switch, typically mounted in the master cylinder or integrated into the hydraulic control unit (HCU).
   • Inspect the switch and its electrical connector for any visible signs of physical damage, corrosion, frayed wires, or loose connections.
   • Trace the wiring harness from the switch back towards the ABS/TCS/ESC module, checking for chafing, pinching, or heat damage.
   • Verify proper brake fluid level and condition; while not a direct electrical cause, it’s a foundational brake system check.
2. OBD-II Scanner & Live Data Analysis:
   • Connect an advanced diagnostic scanner capable of communicating with the ABS/TCS/ESC module.
   • Read all stored Diagnostic Trouble Codes (DTCs), including any pending or historical codes, to identify if other related issues are present.
   • Access the live data stream for the brake pressure switch input. This may appear as a voltage reading (e.g., 0V-5V), a pressure value (e.g., PSI/bar), or a simple ON/OFF state.
   • With the ignition ON and engine OFF (KOEO), observe the brake pressure switch parameter while manually depressing and releasing the brake pedal. The value should change dynamically and consistently. A static reading (e.g., always 0V, always 5V, or no change) strongly indicates a circuit or switch fault.
3. Digital Multimeter (DMM) Testing:
   • Power & Ground Check: With the ignition ON, disconnect the brake pressure switch electrical connector. Using a DMM, check for the specified reference voltage (typically 5V or 12V, consult vehicle wiring diagram) at the appropriate terminal(s) and a stable ground signal at the ground terminal. Readings outside specifications indicate a power or ground supply issue from the control module or wiring.
   • Switch Functionality Test (Out of Circuit): If possible and safe, remove the brake pressure switch. Using a DMM, test its internal resistance or continuity. For a simple ON/OFF switch, it should show a clear change in state (open/closed circuit) when pressure is applied (e.g., by manually depressing the plunger if applicable, or by connecting a vacuum pump/pressure tester if it’s a pressure transducer). For a variable-output sensor, its resistance or voltage output should change proportionally to applied pressure.
   • Wiring Integrity Check: Disconnect the battery. Disconnect both the brake pressure switch connector and the ABS/TCS/ESC module connector. Use the DMM to perform:
     • Continuity Test: Check for continuity (near 0 ohms resistance) on each wire between its respective terminal at the switch connector and the module connector. An infinite resistance reading indicates an open circuit.
     • Short to Ground Test: Check for continuity between each wire and chassis ground. Any reading near 0 ohms indicates a short to ground.
     • Short to Voltage Test: Check for continuity between each wire and battery positive terminal (reconnect battery briefly for this, ensure ignition OFF). Any reading near 0 ohms indicates a short to voltage.
4. Hydraulic Pressure Verification (If necessary): If all electrical checks pass but the code persists, and the switch provides a variable pressure output, it may be necessary to use a hydraulic pressure gauge inserted into the brake line to compare actual pressure against the sensor’s reported value via live data. However, this is rarely needed for a “circuit failure” code.

Recommended Repairs and Solutions

• Replace the Brake Pressure Switch: If DMM tests or live data analysis confirms the brake pressure switch itself is faulty (e.g., incorrect output, no response to pressure changes), replace it with a new OEM-equivalent part. Ensure the new switch is properly torqued and that the brake fluid level is checked and bled if the hydraulic circuit was opened.
• Repair or Replace Damaged Wiring: If the diagnostic process identifies an open, shorted, or corroded wire within the harness, perform a professional wire repair. This involves cutting out the damaged section, soldering in a new section of wire of the correct gauge, and protecting the splice with heat-shrink tubing. Alternatively, replace the entire segment of the harness if damage is extensive. Ensure all connectors are clean and seat properly.
• Clean Corroded Connectors: For issues traced to corroded terminals at the switch or module, carefully clean them using an electrical contact cleaner and a small, non-abrasive brush. Apply dielectric grease to prevent future corrosion before reassembly.
• ABS/TCS/ESC Control Module Replacement: This should be the absolute last resort, only after the brake pressure switch, its wiring, and connectors have been exhaustively tested and ruled out as the cause. Module replacement often requires specialized programming or calibration using a factory-level scan tool specific to the vehicle manufacturer.
• Post-Repair Procedures: After any repair, clear the C1939 code using the diagnostic scanner. Perform a thorough road test under various braking conditions to confirm the repair and ensure the code does not return. Verify that all associated warning lights are off and that affected systems (e.g., cruise control, ABS) function correctly. If the hydraulic system was opened for switch replacement, ensure a proper brake fluid flush and bleeding procedure is performed to remove any air from the system.