C1939 — Brake Pressure Switch Input Circuit Failure #

The definition of this DTC may vary depending on the vehicle manufacturer. Always refer to your OEM (Original Equipment Manufacturer) service documentation for your specific model.

• Ford: Brake Pressure Switch Input Circuit Failure
• Chrysler: Brake Pressure Switch Input Circuit Malfunction
• Land Rover: Brake Pressure Switch Signal Error

Summary of the C1939 Code #

The C1939 code indicates a detected fault in the brake pressure switch input circuit, which is part of your vehicle’s braking system. The brake pressure switch monitors the force applied to the brake pedal and sends data to the vehicle’s electronic control unit (ECU). When the ECU detects an abnormal voltage, open circuit, or erratic input from this switch, it stores the C1939 DTC (Diagnostic Trouble Code) and may illuminate the ABS (Anti-lock Braking System) or brake warning light. This code can affect braking performance and, in some cases, trigger additional safety system warnings. Addressing it promptly is essential for safe vehicle operation.

Common Symptoms for DTC C1939 #

A C1939 code can present a range of symptoms, some more noticeable than others. Most commonly, drivers will see warning lights or experience changes in brake system behavior. Here’s what you might encounter:

• ABS warning light or brake warning light illuminated on the dashboard
• Slightly harder brake pedal feel
• Reduced or disabled ABS function
• Intermittent or non-functioning brake lights
• Potential traction control or stability system warnings

Probable Causes for Code C1939 #

Several issues can trigger the C1939 code, but some are more common than others. Here are the most frequent causes, listed in order of likelihood:

1. Faulty brake pressure switch (the component that detects pedal force and sends signals to the ECU)
2. Wiring problems (damaged, corroded, or disconnected wires in the switch circuit)
3. Poor electrical connection at the brake pressure switch connector
4. Defective connector (loose, bent, or corroded pins)
5. Faulty or damaged ABS control module (the ECU responsible for processing brake data)

Step-by-Step Diagnostic Guide for C1939 #

Diagnosing the C1939 code requires a methodical approach to pinpoint the exact cause. Here’s a recommended procedure:

• Step 1: Retrieve all stored and pending codes using a professional-grade scan tool. Note any related brake or ABS codes.
• Step 2: Inspect the brake pressure switch and its connector for visible damage, corrosion, or disconnection.
• Step 3: Check the wiring harness between the switch and the ABS control module for damage, pinched wires, or exposed conductors.
• Step 4: Use a digital multimeter to test for correct voltage and ground at the switch connector, following OEM wiring diagrams.
• Step 5: If wiring and power are good, test the brake pressure switch itself for continuity and resistance per manufacturer specifications.
• Step 6: If the switch and wiring are confirmed good, consider possible ABS module faults and follow OEM diagnostic instructions.
• Step 7: After repairs, clear codes and perform a road test to verify the fix.

Possible Repairs for C1939 #

Once the root cause is found, repairs usually target the faulty component or connection. Here are the most common solutions:

• Replace the faulty brake pressure switch
• Repair or replace damaged wiring in the switch circuit
• Clean, repair, or replace corroded or loose connectors
• Replace the ABS control module if confirmed faulty (rare)

Is It Safe to Drive with Code C1939? #

Driving with a C1939 code is not recommended. While some vehicles may only limit ABS or traction control functions, others can experience reduced braking performance or unpredictable brake response. Even if braking seems normal, the loss of ABS or related safety features increases the risk of skidding or losing control during emergency stops or on slippery roads. Always repair brake-related issues promptly to maintain safe operation.

Vehicles Most Commonly Affected by C1939 #

Although the C1939 code can appear on a variety of vehicles, it is most frequently reported on certain OEM models due to design similarities or known component wear patterns. Vehicles where this code is commonly found include:

• Ford Fusion, Focus, and Escape (various model years)
• Chrysler Town & Country, 300, and Pacifica
• Land Rover Range Rover and Discovery
• Jeep Grand Cherokee
• Dodge Caravan

Common Mistakes & Best Practices for C1939 #

When troubleshooting code C1939, certain errors can prolong the repair or lead to incorrect part replacements. Avoid these pitfalls by following proven best practices:

• Replacing the brake pressure switch without first checking for wiring or connector faults
• Ignoring related codes that could indicate a broader issue with the ABS system
• Overlooking corrosion or water intrusion at connectors
• Not using OEM wiring diagrams for accurate circuit testing
• Failing to clear codes and retest after repairs

Express FAQ for C1939 #

Here are quick answers to some of the most common questions about the C1939 code:

• Q: Can a bad brake pressure switch affect cruise control?
• A: Yes, many vehicles use the brake pressure switch to disengage cruise control. A failure can disable this feature as a safety measure.
• Q: Will the C1939 code reset itself after repairs?
• A: In most cases, you’ll need to clear the code with a scan tool after repairs. Some vehicles may eventually clear it automatically after several drive cycles if the fault is resolved.