U0101

Code

HYUNDAI U — Network/User

Brand: HYUNDAI

Type: U — Network/User

Views: UK: 25 EN: 44 RU: 60

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in CAN bus wiring between modules
Corroded, loose, or damaged connector at the TCM or related modules
Blown fuse or lost power/ground to the TCM
TCM internal fault or failed module
Network termination fault (missing/shorted 120Ω termination, abnormal bus resistance)
Low battery voltage or poor battery connection during key ON

Symptoms

MIL (Malfunction Indicator Lamp) or AT warning lamp illuminated
Transmission enters limp/limited shift mode or fails to shift normally
No communication with TCM when using scan tool (TCM not present)
Erratic transmission behavior or incorrect gear display
Other modules report CAN communication errors or multiple U‑codes

What to check

Read and record all U‑codes and freeze frame data with a capable scan tool
Check battery voltage (should be ≥12.4 V at rest) and verify good battery terminals
Visually inspect TCM connectors and wiring for corrosion, bent pins, water damage, or rodent chew
Check fuses and relays supplying power to the TCM and ECM/PCM
Using a digital multimeter, measure voltage and continuity on the TCM power and ground pins
With key ON, engine OFF, measure CAN bus bus idle voltages at a known good junction: CAN H ≈2.5 V, CAN L ≈2.5 V (see signal parameters)

Signal parameters

Idle (recessive) CAN H ≈ 2.5 V, CAN L ≈ 2.5 V (measured to chassis ground)
Dominant state: CAN H rises ~3.5 V, CAN L falls ~1.5 V (dynamic)
Typical CAN bus termination: two 120 Ω resistors in parallel → ~60 Ω across H and L with ignition OFF
Common CAN bus speed on modern Hyundai C‑CAN: typically 500 kbps (verify vehicle-specific)

Diagnostic algorithm

Connect a diagnostic scan tool and confirm U0101 and note any other U‑codes. Attempt to communicate with TCM; document failure behavior.
Verify battery voltage and health. Recharge or replace battery if low; clear codes and attempt communication again.
Inspect visible TCM connector and harness for damage, corrosion or water. Repair or clean as needed. Check that connector is fully seated.
Check fuses and relays for TCM power supply; verify +12V at TCM power pin(s) with ignition ON and good ground at chassis/TCM ground pin.
With key ON, use meter to measure CAN H and CAN L voltages at TCM connector and a known good junction. Compare to expected signal parameters.
With ignition OFF, measure resistance between CAN H and CAN L at one point — expect ≈60 Ω. If open or abnormal, isolate sections by disconnecting modules one at a time and rechecking resistance to find the fault segment.
Check for shorts from CAN H/L to battery + or ground using an ohmmeter and backprobe points. Repair damaged wiring or replace pigtails as required.
If wiring and power/ground are good but communication still fails, try swapping or bench‑testing the TCM (if available) or connecting a known‑good module to isolate a module fault. Reprogram/initialize replacement modules per manufacturer procedure.
After repairs, clear codes, perform communication check with scan tool, test drive to confirm normal transmission behavior and monitor for reappearance of code.

Likely causes

Loose/damaged connector or pin at the TCM or a nearby junction
Open/short in CAN H or CAN L between TCM and other control modules
Blown fuse or missing +12V/ground to the TCM
Faulty TCM or corrupted software/programming

Fault status

⚠️ Status

Lost communication with the Transmission Control Module (TCM) on the vehicle CAN network (U0101). Diagnostic communication to the TCM is not available.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours

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U0101 — Lost Communication with TCM | Lost Communication With TCM | Serial Communication Problem With TCM | Lost Communication With ECM/PCM ‘A’ (C-CAN) | Lost Communication with TCM (C-CAN) | Lost Communication with ECM/PCM ‘A’