U1900

CAN Communication Bus Fault

Network / Communication Network/Communication CAN Bus Communication 🟡 Moderate — Fix within a week ⚠️ Drive with Care
💬

What This Actually Means

In plain language — no jargon

Your vehicle's computers can't talk to each other properly over the CAN bus—think of it like a broken telephone line between the engine, transmission, and other systems. Without communication, the vehicle may run poorly or not at all.

⚡

Symptoms You May Notice

3 known symptoms for this code
Check Engine Light illuminated
Vehicle won't start or starts intermittently
Multiple unrelated fault codes stored simultaneously
🔬

How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors the CAN bus for valid data signals between modules. It checks for proper voltage levels, message timing, and data integrity across the network. When bus voltage drops, signal timing fails, or messages become corrupted, the ECU sets this fault.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
CAN Bus Voltage 2.5V to 3.5V (dominant state) <2.0V or >3.8V sustained
Message Timeout Expected module responses received No response from modules >500ms
🔧

Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
CAN Bus Connectors and Harness
Inspect for loose, corroded, or damaged connectors at the OBD-II port and under the dashboard; reseat firmly.
2
Battery Terminals
Clean corrosion from battery terminals and cable ends with a wire brush and ensure tight connections.
3
CAN Bus Terminating Resistors
Have a technician verify terminating resistors at each end of the CAN loop (120Ω) with a multimeter if wiring is intact.
⚠️

When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code U1900 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
🔄

How to Clear Code U1900

What happens after you fix the fault

Once the fault is repaired, U1900 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.