Code
U0001
ISUZU
U — Network/User
CAN-Bus Malfunction.15E-92
AI status
Completed
Completed
100%
Causes
- Open, shorted or chafed CAN wiring (CAN_H / CAN_L)
- Corroded or loose connectors or pins at modules or junctions
- Failed or powered-off control module (bus-off)
- Missing or damaged termination resistor(s)
- Short to battery or ground on CAN lines
- Low battery voltage or poor ground(s) affecting module operation
Symptoms
- MIL (check engine light) or multiple warning lamps illuminated
- One or more modules not present or reporting errors on a scan tool
- Loss of specific functions (e.g., ABS, instrument cluster, transmission communication)
- Intermittent or permanent no-start if powertrain modules are not communicating
- Erratic or frozen instrument cluster messages, or flickering gauges
- Communication-related codes alongside U0001
What to check
- Connect a capable scan tool and read all U- and B- codes; note which modules are missing or reporting errors
- Visually inspect CAN connectors, junction blocks and wiring for corrosion, damage, loose pins or water ingress
- Check battery voltage and main engine/ chassis grounds for proper connections and corrosion
- Using a DMM, measure resistance between CAN_H and CAN_L across the bus — expect ~60 Ω total (two 120 Ω terminators in parallel)
- Check for bus-off status or error counters in module data with a scanner
- Backprobe CAN_H and CAN_L with an oscilloscope (preferred) or DMM to check voltages and waveform integrity
Signal parameters
- Recessive (idle) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each)
- Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
- Termination resistance: ≈ 60 Ω measured between CAN_H and CAN_L (two 120 Ω terminators in parallel)
- Common bit rates: 125 kbps, 250 kbps or 500 kbps — verify vehicle specification
- Bus should show clean square/triangular differential waveform on oscilloscope; excessive ringing/noise indicates wiring or grounding issues
Diagnostic algorithm
- Retrieve and record all DTCs, freeze frame and module list with a factory-level scan tool. Note which modules are not communicating.
- Visually inspect all CAN-related connectors, junctions and wiring for obvious damage, corrosion, bent pins or water. Repair any issues found.
- Verify battery voltage (engine off and ON) and all major ground points are clean and tight. Repair/clean as needed.
- With ignition ON, measure resistance between CAN_H and CAN_L at a convenient connector — expect ~60 Ω. If open or far from 60 Ω, locate missing/open terminator or break in harness.
- Backprobe CAN_H and CAN_L at a known good connector and check DC voltages: both ~2.5 V at idle. If voltages are incorrect, isolate the source (short/low/high).
- If available, use an oscilloscope to observe CAN waveforms while operating modules; look for proper differential levels and clean transitions. Identify noisy or collapsed signals.
- Isolate suspect modules by disconnecting nodes one at a time while watching the bus on the scanner/oscilloscope to find which disconnection clears the fault.
- Repair or replace damaged wiring, connectors, or the failing module as indicated by isolation tests. Replace missing or damaged terminators.
- Clear codes and verify repair by cycling ignition and re-scanning; test-drive and re-check for return of U0001 or related codes.
- If intermittent after repairs, perform extended monitoring and consider harness replacement in areas prone to chafing or water intrusion.
Likely causes
- Faulty connector or pin corrosion at a gateway/module
- Broken or shorted CAN_H or CAN_L wiring near harness junctions
- Failed control module placed into bus-off state
- Missing or open termination resistor(s) at bus ends
- Intermittent ground or battery supply to one or more modules
Fault status
Status
U0001 indicates a detected malfunction in the vehicle CAN communication bus: messages are not being properly transmitted or received. The fault can be caused by wiring/connectors, termination problems, a failing module (bus-off), low power or grounding issues. Further isolation required to identify the specific fault location.
Repair difficulty: Medium
Diagnostic time: 1.0-4.0 hours
Similar codes
Repair manuals
Manual library for ISUZU
229
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