U2104

Code

OPEL U — Network/User

CAN-Bus reset counter overrun

Brand: OPEL

Type: U — Network/User

Views: UK: 2 EN: 5 RU: 6

AI status

Completed

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Page language: EN

Causes

Intermittent or shorted CAN wiring (CANH/CANL)
Poor or intermittent ground or battery supply to one or more modules
Failing or improperly powered control module on the CAN network
Corroded/loose connectors or damaged pin(s) in network connectors
Incorrect termination (missing or shorted 120 Ω resistors)
Excessive bus load or electrical noise (aftermarket devices, high current loads)

Symptoms

MIL/Warning lights illuminated related to communication faults
Intermittent or permanent loss of functions controlled by one or more modules (doors, ABS, engine, instrument cluster)
Multiple communication-related DTCs stored in one or more modules
Vehicle behaves normally after power cycle but fault returns
Recorded high error or reset counters in module data

What to check

Connect a professional scan tool and read all stored and pending U-codes and freeze-frame data
Check for other communication DTCs (U0100, U0073, U0121, etc.) and note affected modules
Inspect battery and chassis grounds for corrosion, tightness and correct connection
Visually inspect CAN wiring, harnesses and module connectors for damage, corrosion, or pushing pins
Measure CAN termination resistance across CANH–CANL (approx. 60 Ω total) with battery disconnected
Use oscilloscope or CAN bus analyzer to capture CANH/CANL waveforms and look for noise, missing recessive/dominant states, or repeated bus resets

Signal parameters

CAN bus speeds commonly used: 125 kbps, 250 kbps, 500 kbps (verify vehicle-specific speed)
CAN idle (recessive) common-mode ~2.5 V on CANH and CANL
Dominant state example: CANH ~3.5 V, CANL ~1.5 V (differential ~2.0 V)
Termination resistance: two 120 Ω resistors in parallel ≈ 60 Ω across CANH–CANL
Watch for error counters and reset counters exposed by module live data
Typical message timing: check for repeated re-initialisation frames or firmware-triggered reboots

Diagnostic algorithm

Retrieve all communication-related codes and freeze-frame/live data from all modules; record affected modules and time stamps.
Check battery health and measure supply/gnd voltages at affected modules while reproducing the fault; repair any supply or ground issues.
Perform a visual inspection of wiring, connectors and modules on the affected CAN segment; repair any damage or corrosion.
Measure termination resistance across CANH–CANL with ignition off; expected ~60 Ω. If out of range, locate missing/shorted terminator(s).
Use a CAN bus analyzer or oscilloscope to monitor traffic and verify correct recessive/dominant voltages, bus speed and to capture reset events. Look for noise, message storms, or a module repeatedly sending reset/init frames.
If a suspect module is identified by analysis, disconnect it (with ignition off and following safety/manufacturer procedures) and repeat monitoring to see if resets stop. Alternatively isolate network segments to narrow the fault location.
Check for and remove recent aftermarket modules or accessories that could affect the bus.
If wiring and supply are good and a single module is implicated, check for available software/firmware updates for modules; reprogram or replace the module per manufacturer guidance.
Clear codes and road-test to confirm the issue is resolved; re-scan to ensure no recurrence. Document findings and repairs.

Likely causes

Faulty module repeatedly resetting and flooding the bus
Open/short/low-quality connector on a CAN segment causing repeated reinitialisation
Missing or open termination resistor (incorrect 60 Ω across CANH–CANL)
Battery/ignition supply dropout to a module leading to repeated ECU restarts

Fault status

⚠️ Status

CAN bus reset counter overrun — module detected excessive CAN reset/re-initialisation events on the vehicle network; communication is unstable or repeatedly reinitialising.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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U2104 — CAN-Bus reset counter overrun