What Does U0004 Mean?
The CAN High (+) circuit voltage is below normal operating range, typically indicating a short to ground or excessive current draw. The bus cannot maintain proper voltage differential for reliable communication.
Safety Warning
Complete loss of network communication will disable multiple vehicle systems including safety features. Vehicle should not be driven until repaired.
Common Causes
45%
CAN High wire shorted to ground or chassis
25%
Failed control module with internal short on CAN High pin
15%
Damaged connector causing short to ground
10%
Water intrusion causing short circuit in connector or module
5%
Incorrect repair or wiring modification shorting circuit
Diagnostic Steps
1
Step 1: Measure voltage on CAN High circuit; if below 2.0V with key on, confirm short to ground condition
2
Step 2: Disconnect modules one at a time while monitoring CAN High voltage to isolate module causing short
3
Step 3: Check resistance from CAN High to ground with key off and modules disconnected; should read infinite resistance
4
Step 4: Inspect CAN High wire for damage where harness contacts sharp edges, heat sources, or moving components
5
Step 5: Check for water intrusion in connectors and modules, especially those in engine bay or under vehicle
Estimated Repair Cost
$150 - $1,500
Parts + labor, varies by vehicle and location
The CAN High (+) circuit voltage is below normal operating range, typically indicating a short to ground or excessive current draw. The bus cannot maintain proper voltage differential for reliable communication.
The most common cause of U0004 (High Speed CAN Communication Bus (+) Low) is: CAN High wire shorted to ground or chassis
Typical repair costs for U0004 range from $150 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Complete loss of network communication will disable multiple vehicle systems including safety features. Vehicle should not be driven until repaired.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0004 to identify the root cause.
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Quick Info
Category
Network
System
Network Communication
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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