OBD Code U1900: CAN Communication Bus Malfunction

Quick Answer: Code U1900 indicates a failure in the CAN (Controller Area Network) communication bus, preventing vehicle modules from talking to each other. The most common fix involves checking and repairing wiring connections, replacing the CAN bus terminating resistors, or repairing a faulty module.

The U1900 diagnostic trouble code is a manufacturer-specific code that signals a malfunction in your vehicle’s CAN (Controller Area Network) communication bus. This is the “nervous system” of your vehicle—a network that allows different electronic control modules (like the engine control unit, transmission control unit, and body control module) to communicate with each other. When this code appears, it means the modules aren’t talking properly, which can cause various electrical and performance issues throughout the vehicle.

What Does U1900 Mean?

U1900 is a network communication code that falls under the “U” series of diagnostic trouble codes, which are reserved for manufacturer-specific network communication problems. Unlike standardized codes, U1900 may have slightly different meanings depending on your vehicle’s manufacturer (Ford, GM, Toyota, BMW, etc.), but the underlying issue is always the same: the CAN bus communication system is experiencing a malfunction.

The CAN bus operates at high speed (typically 500 kbps) and uses two twisted-pair wires to transmit data between modules. When the system detects errors in message transmission, reception, or bus integrity, it triggers the U1900 code. This could mean:

A break or short in the CAN bus wiring
A faulty CAN bus terminating resistor
A defective module unable to communicate
Corroded or loose connectors on the CAN bus
Voltage supply issues to the CAN bus circuit

Common Symptoms

Drivers experiencing U1900 may notice one or more of the following symptoms:

Check Engine Light (CEL) – The primary indicator that the code has been triggered
Multiple warning lights – ABS, airbag, transmission, stability control, and other system lights may illuminate simultaneously
Loss of power steering – If the steering module loses communication
Transmission shifting problems – Harsh shifts, delayed engagement, or limp mode activation
Engine performance issues – Rough idle, hesitation, or reduced power
Instrument cluster malfunctions – Gauges not responding, display errors, or blank screens
Cruise control failure – System becomes inoperative
Climate control malfunction – HVAC system unresponsive
Door lock/window failures – Body control module communication loss
Vehicle may enter limp mode – Reduced performance as a safety precaution

Possible Causes (Ranked by Frequency)

Understanding the most common causes helps guide your diagnostic approach:

1. Corroded or Loose CAN Bus Connectors

This is the most frequent cause. Moisture, salt, and dirt can corrode the CAN bus connector pins, preventing proper electrical contact. Loose connectors due to vibration or improper installation are also common culprits.

2. Damaged CAN Bus Wiring

The CAN bus uses two wires (CAN High and CAN Low) that run throughout the vehicle. Damage from rodents, pinched wires during repairs, or wear from vibration can break continuity or cause shorts.

3. Faulty CAN Bus Terminating Resistors

CAN bus networks require 120-ohm terminating resistors at both ends of the bus. If these resistors fail or become disconnected, the bus cannot function properly. This is particularly common in vehicles with aftermarket modifications.

4. Defective CAN Bus Module

A faulty engine control unit (ECU), transmission control module (TCM), body control module (BCM), or other networked module can prevent communication. This is less common but requires module replacement.

5. Voltage Supply Issues

The CAN bus circuit requires stable 12V power. A failing alternator, weak battery, or faulty voltage regulator can cause communication failures.

6. Aftermarket Device Installation

Improperly installed aftermarket stereos, GPS units, or diagnostic devices can interfere with CAN bus communication or damage the wiring.

7. Water Intrusion or Moisture

Water in electrical connectors, especially in areas prone to flooding or high humidity, can cause shorts and communication failures.

Diagnostic Steps

Follow these steps to diagnose the U1900 code:

Step 1: Scan for Additional Codes

Use a quality OBD-II scanner to read all stored and pending codes. U1900 rarely appears alone—other codes will help pinpoint the affected module or system. Document all codes before proceeding.

Step 2: Visual Inspection

Perform a thorough visual inspection of:

All CAN bus connectors (typically located in the engine bay, under the dashboard, and behind door panels)
Look for corrosion, moisture, or bent pins
Check for damaged, pinched, or cut wiring
Inspect for loose connectors or disconnected modules
Look for signs of rodent damage or water intrusion

Step 3: Check Battery and Voltage

Use a multimeter to verify:

Battery voltage (should be 12.6V at rest, 13.5-14.5V while running)
Voltage at the CAN bus connectors (should be stable at 12V)
Ground connections are clean and tight

Step 4: Inspect CAN Bus Wiring

Using a multimeter in continuity mode:

Test CAN High wire continuity from one end of the bus to the other
Test CAN Low wire continuity
Check for shorts between CAN High and CAN Low (resistance should be very high)
Typical CAN bus resistance values: 60 ohms (both wires in parallel)

Step 5: Test Terminating Resistors

Locate the CAN bus terminating resistors (typically 120 ohms each, found at both ends of the bus):

Use a multimeter to verify resistance values
Check for continuity and proper connection
Replace if readings are incorrect or open circuit

Step 6: Module Communication Test

Using an advanced diagnostic scanner:

Attempt to communicate with each module individually
Identify which modules are responding and which are not
This narrows down whether the problem is a wiring issue or a faulty module

Step 7: Clear and Retest

After making any repairs:

Clear the code using your scanner
Drive the vehicle through various conditions (city, highway, idle)
Rescan to confirm the code does not return

Repair Cost Estimates

The cost to repair U1900 varies widely depending on the root cause:

Connector cleaning/repair: $50–$150 (DIY) or $150–$300 (shop labor)
CAN bus wiring repair: $200–$600 (shop labor, depending on location and extent of damage)
Terminating resistor replacement: $100–$400 (parts and labor)
Module replacement: $500–$2,000+ (ECU, TCM, or BCM replacement can be expensive, especially if reprogramming is required)
Diagnostic service: $100–$200 (many shops charge this upfront, which may be credited toward repairs)

Total average repair cost: $300–$800 for most common causes; $1,500–$3,000+ if a module replacement is needed.

Can I Still Drive?

Severity: Moderate to High

Whether you can safely drive with U1900 depends on which systems are affected:

Safe to drive (short distances): If only non-critical systems like the infotainment or climate control are affected, you may drive to a repair shop, but avoid highway speeds.
Not safe to drive: If the code affects the engine control, transmission, brakes, steering, or airbag systems, do not drive the vehicle. These are critical safety systems.
Limp mode: Many vehicles will enter limp mode to protect the engine and transmission, severely limiting performance.

Recommendation: Have the vehicle scanned immediately to determine which modules are affected. If safety-critical systems are involved, arrange for professional diagnosis and repair before driving.

Frequently Asked Questions

Can I fix U1900 myself?

If the cause is a loose connector or corroded pins, you may be able to clean and reseat connectors yourself. However, if the problem involves wiring repair, module replacement, or reprogramming, professional diagnostic equipment and expertise are required. Always start with a visual inspection and connector cleaning before seeking professional help.

Will U1900 go away on its own?

No. U1900 indicates a real communication problem that will not resolve without repair. While the code may occasionally clear after a restart if the issue is intermittent, it will return until the underlying cause is fixed. Ignoring it can lead to cascading failures in other vehicle systems.

Is U1900 the same on all vehicles?

U1900 is a manufacturer-specific code, so its exact meaning and the systems it affects may vary slightly between Ford, GM, Toyota, BMW, and other manufacturers. However, the root cause is always a CAN bus communication malfunction. Always consult your vehicle’s specific service manual for precise definitions and diagnostic procedures.

What should I do if U1900 appears after an accident or electrical work?

If the code appeared after a collision, water damage, or recent electrical repairs, the CAN bus wiring or connectors may have been damaged. Start with a visual inspection of the affected area, check for loose connectors, and verify wiring continuity. If you recently had work done, contact the repair facility to ensure proper CAN bus connections were maintained.