C0939 - Understanding DTC C0939: Common Issues in 2015-2018 Ford F-150 and More

Quick Answer

C0939 means: DTC C0939 indicates a communication issue with the Transmission Control Module (TCM).

Fix: Check and replace faulty components Cost: $800 - $1200 Time: 90-180 minutes

Can I drive with C0939? Address this issue immediately to prevent further transmission damage.

Common Questions

What does C0939 mean and how does it affect my car?

DTC C0939 indicates a communication problem within the vehicle's Transmission Control Module (TCM). This can lead to poor shifting performance, reduced fuel efficiency, and potentially severe transmission damage if not addressed. It's essential to diagnose and repair the issue promptly to maintain optimal vehicle performance.

What are the most common causes of C0939 and how much does it cost to fix?

Common causes of C0939 include a faulty TCM, wiring issues, or low transmission fluid levels. Repair costs can vary, with TCM replacement ranging from $800 to $1200 and wiring repairs costing between $150 and $400. A professional inspection is recommended to determine the exact cause.

Can I drive my car with C0939 or should I stop immediately?

While you might be able to drive with DTC C0939, it's not advisable. The vehicle may exhibit erratic behavior or face further transmission damage. If you experience severe symptoms like slipping gears, it’s best to stop driving and seek professional help.

How can I diagnose C0939 myself using GeekOBD APP?

To diagnose C0939 using the GeekOBD APP, connect the app to your vehicle's OBD2 port. Run a full scan to retrieve the trouble codes and check the freeze frame data. Look for patterns in the data to identify potential causes and refer to the app's troubleshooting guide for further assistance.

What vehicles are most commonly affected by C0939?

DTC C0939 is commonly found in vehicles like the 2015-2018 Ford F-150, Chevrolet Silverado, and Ram 1500. These models often utilize similar electronic transmission systems, making them susceptible to this trouble code.

How can I prevent C0939 from happening again?

To prevent DTC C0939, ensure regular maintenance of your transmission, including fluid changes and inspections of wiring and connectors. Additionally, stay updated with software updates from manufacturers, as these can resolve known issues before they escalate.

What is C0939?

DTC C0939 is a diagnostic trouble code that indicates a problem with the vehicle's high-speed communication bus, specifically relating to the Transmission Control Module (TCM). This code is particularly prevalent in vehicles like the 2015-2018 Ford F-150 and other models utilizing similar electronic transmission systems. When this code is triggered, it often means that the TCM is unable to communicate effectively with other modules, which can lead to a variety of performance issues, including erratic shifting, poor fuel economy, and even potential transmission failure if not addressed promptly. Car owners may notice that their vehicle struggles to shift gears smoothly or experience sudden power losses during acceleration. Ignoring this code can lead to more severe transmission issues, resulting in costly repairs down the line. Therefore, understanding the implications of DTC C0939 is crucial for maintaining the health and performance of your vehicle. If you see this code, it’s a clear sign that a professional evaluation is necessary, and addressing it early can save you from bigger problems in the future.

System: C - Chassis (ABS, Steering, Suspension)

Symptoms

Common symptoms when C0939 is present:

  • The check engine light stays on continuously, indicating an underlying issue that needs to be diagnosed.
  • You may notice the engine hesitates during acceleration, which can make driving feel unsafe and unresponsive.
  • Fuel economy decreases significantly, often by 10-15%, making your trips more expensive.
  • The transmission may slip or hesitate between gears, leading to a jerky driving experience.
  • In some cases, you could experience unexpected shifts or a complete loss of power during driving.

Possible Causes

Most common causes of C0939 (ordered by frequency):

  1. The most common cause of DTC C0939 is a faulty Transmission Control Module, which occurs in about 40% of cases. This can be due to age, wear, or manufacturing defects.
  2. A second common cause is damaged wiring or connectors in the transmission harness, which can disrupt communication between the TCM and other vehicle modules.
  3. Another potential cause is low transmission fluid levels or contamination, which can lead to erratic TCM behavior. Regular maintenance can help prevent this issue.
  4. Less common but serious causes include a malfunctioning engine control unit (ECU), which may require extensive diagnostic tests to confirm.
  5. A rare cause could be factory software glitches. In such cases, an update or reprogramming of the TCM may be necessary.

C0939 Repair Costs

Cost Breakdown by Repair Type

Transmission Control Module Replacement

Replacing the faulty TCM with a new or refurbished unit.

  • Total: $800 - $1200
  • Success rate: 85%
Wiring Repair

Repairing or replacing damaged wiring or connectors in the TCM harness.

  • Total: $150 - $400
  • Success rate: 75%
Software Update

Updating the transmission control software to rectify communication issues.

  • Total: $100 - $300
  • Success rate: 90%
Money-Saving Tips for C0939
  • Start with the most common and least expensive repairs first
  • Use GeekOBD APP to confirm diagnosis before replacing expensive parts
  • Consider preventive maintenance to avoid future occurrences
  • Compare prices for OEM vs aftermarket parts based on your needs
  • Address the issue promptly to prevent more expensive secondary damage

Diagnostic Steps

Professional C0939 Diagnosis Process

Follow these systematic steps to accurately diagnose C0939. Each step builds on the previous one to ensure accurate diagnosis.

Step 1: Step 1: Perform an initial visual inspection to check for obvious signs of damage such as frayed wiring, loose connections, or corrosion around the TCM and transmission harness (5-10 minutes)

Step 1: Perform an initial visual inspection to check for obvious signs of damage such as frayed wiring, loose connections, or corrosion around the TCM and transmission harness (5-10 minutes).

GeekOBD APP Tip: Use GeekOBD APP to monitor real-time data during this diagnostic step for accurate results and professional-grade analysis.
Step 2: Step 2: Use an OBD2 scanner, such as the GeekOBD APP, to retrieve all diagnostic codes and freeze frame data

Step 2: Use an OBD2 scanner, such as the GeekOBD APP, to retrieve all diagnostic codes and freeze frame data. This will give context to the fault and help pinpoint the issue (10-15 minutes).

GeekOBD APP Tip: Use GeekOBD APP to monitor real-time data during this diagnostic step for accurate results and professional-grade analysis.
Step 3: Step 3: Conduct component testing

Step 3: Conduct component testing. Utilize a multimeter to check the voltage and ground of the TCM and associated wiring to ensure they are functioning properly (20-30 minutes).

GeekOBD APP Tip: Use GeekOBD APP to monitor real-time data during this diagnostic step for accurate results and professional-grade analysis.
Step 4: Step 4: After any repairs, perform a system function test to verify that everything operates correctly and clear the codes from the vehicle’s computer (10-15 minutes)

Step 4: After any repairs, perform a system function test to verify that everything operates correctly and clear the codes from the vehicle’s computer (10-15 minutes).

GeekOBD APP Tip: Use GeekOBD APP to monitor real-time data during this diagnostic step for accurate results and professional-grade analysis.
Step 5: Step 5: Finally, conduct a road test under various driving conditions to confirm that the repair resolved the issue without returning the DTC (15-20 minutes)

Step 5: Finally, conduct a road test under various driving conditions to confirm that the repair resolved the issue without returning the DTC (15-20 minutes).

GeekOBD APP Tip: Use GeekOBD APP to monitor real-time data during this diagnostic step for accurate results and professional-grade analysis.

Important Notes

  • Always verify the repair with GeekOBD APP after completing diagnostic steps
  • Clear codes and test drive to ensure the problem is resolved
  • Address underlying causes to prevent code recurrence

Real Repair Case Studies

Case Study 1: Transmission Control Module Replacement on 2017 Ford F-150

Vehicle: 2017 Ford F-150, 65,000 miles

Problem: Customer reported erratic shifting and check engine light illumination.

Diagnosis: After scanning with GeekOBD, DTC C0939 was retrieved. Visual inspection showed corroded wiring leading to the TCM.

Solution: Replaced the corroded wiring and installed a new TCM. Conducted a software update to ensure compatibility.

Cost: $1,100 (parts: TCM - $800, wiring - $300)

Result: The vehicle shifted smoothly post-repair, and follow-up tests showed no recurrence of the DTC.

Case Study 2: Wiring Repair on 2015 Chevrolet Silverado

Vehicle: 2015 Chevrolet Silverado, 80,000 miles

Problem: Owner experienced loss of power and check engine light.

Diagnosis: DTC C0939 was confirmed. Inspection revealed frayed wiring in the transmission harness.

Solution: Repaired the damaged wiring and cleared the codes. No parts needed replacement.

Cost: $250 (labor only)

Result: The vehicle returned to normal operation, with no further issues reported.

Diagnose C0939

Use GeekOBD APP for professional diagnosis!

  • Real-time data monitoring
  • Advanced diagnostic features
  • Step-by-step repair guidance
  • Professional-grade analysis
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Code Information

Code: C0939
System: Chassis (ABS, Steering, Suspension)
Severity: HIGH
Category: Chassis Codes