What Does U0021 Mean?
The positive (CAN High) wire of the low-speed CAN bus has an open circuit, breaking the electrical path. This prevents proper differential signaling and disrupts communication between modules on the low-speed network.
Common Causes
35%
Broken or severed CAN High wire in harness
30%
Corroded or damaged connector pins causing open circuit
20%
Failed module with internal open on CAN High circuit
10%
Wire damage from rodent activity or harness wear
5%
Disconnected or improperly seated connector
Diagnostic Steps
1
Step 1: Measure continuity of CAN High circuit from gateway module to each module on low-speed network
2
Step 2: Inspect all connectors for loose connections, backed-out pins, or corrosion causing open circuits
3
Step 3: Check for wire breaks by gently flexing harness while monitoring continuity with multimeter
4
Step 4: Verify termination resistors are present and connected properly at network endpoints
5
Step 5: Use oscilloscope to verify CAN High signal is present where continuity exists to isolate fault location
Estimated Repair Cost
$150 - $1,500
Parts + labor, varies by vehicle and location
The positive (CAN High) wire of the low-speed CAN bus has an open circuit, breaking the electrical path. This prevents proper differential signaling and disrupts communication between modules on the low-speed network.
The most common cause of U0021 (Low Speed CAN Communication Bus (+) Open) is: Broken or severed CAN High wire in harness
Typical repair costs for U0021 range from $150 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Low-speed network controls non-critical systems primarily. Vehicle can typically be driven safely but comfort and convenience features will be affected. Repair soon to restore full functionality.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0021 to identify the root cause.
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Quick Info
Category
Network
System
Network Communication System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
