U0100

Code

MERCEDES-BENZ U — Network/User

Lost Communication with ECM/PCM A

Brand: MERCEDES-BENZ

Type: U — Network/User

Views: UK: 45 EN: 66 RU: 65

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or shorted CAN bus wiring (CAN High / CAN Low)
ECM/PCM power supply or ground failure (fuse, relay, connector)
Faulty ECM/PCM or failed CAN transceiver
Poor connector contact or corrosion at ECM or splice
Missing/incorrect termination resistor or damaged terminators
Aftermarket device or module loading/disturbing the CAN bus

Symptoms

MIL/ABS/traction/other dash warning lamps illuminated; network-related messages
Engine may not start or runs poorly if ECM is offline
Loss of vehicle functions controlled by ECM (cruise, engine management, transmission control dependent on ECM input)
Multiple communication-related DTCs on other control units
No ECM data/live parameters available with scan tool

What to check

Read and record all stored and pending DTCs and freeze frame using a capable scan tool
Verify battery voltage steady (12.0–14.5 V) and good battery condition
Check fuses and relays powering the ECM and CAN bus (replace if blown)
Visually inspect ECM connector, wiring harness, and CAN bus splices for damage, corrosion, or water intrusion
Check for other modules reporting communication errors (confirms network vs single module)
Measure CAN bus termination resistance between CAN_H and CAN_L with ignition OFF (approx. 60 Ω expected)

Signal parameters

Battery voltage at ECM: nominal 11–14.5 V (engine off/on respectively)
ECM ground resistance:
CAN_H idle voltage: approx. 2.5 V (recessive state)
CAN_L idle voltage: approx. 2.5 V (recessive state)
CAN_H dominant state: ~3.5 V; CAN_L dominant state: ~1.5 V (differential ~2.0 V)
Termination resistance CAN_H to CAN_L (ignition OFF): ~60 Ω (two 120 Ω resistors in parallel)

Diagnostic algorithm

Verify the exact U0100 freeze frame and confirm this is an active/current code. Record time and conditions.
Scan all modules to list other U‑codes or communication faults. If multiple modules show network off, suspect bus or power/ground.
Inspect battery, main fuses and ECM power/ground connections. Repair any low voltage or high resistance grounds.
Visually inspect wiring, harness rub points, and connectors on the ECM and along CAN bus routes. Repair damaged wiring or corroded pins.
With a DVOM, measure CAN_H and CAN_L voltages and measure CAN_H–CAN_L termination resistance (~60 Ω).
Use an oscilloscope or CAN bus analyzer to look for valid CAN frames, noise, or a dominant/short condition. Identify noisy node or shorted branch.
If a suspect module or aftermarket device is on the same branch, disconnect it and recheck bus function.
If wiring and bus appear good but ECM does not respond, verify ECM supply and ground under load. If supply/ground ok, consider ECM CAN transceiver failure.
Check for required software updates or coding after replacement; reflash/reprogram if software corruption or mismatch is suspected.
After repairs, clear codes, perform a road test and re-scan to confirm the U0100 does not return.

Likely causes

Broken or chafed CAN wiring between ECM and other modules
Blown fuse or relay powering ECM
Poor ground at ECM chassis/ground strap
Intermittent connector pin or corrosion at ECM connector
Failed CAN transceiver inside the ECM (internal hardware failure)
Incorrect module coding or incomplete programming after replacement

Fault status

⚠️ Status

Lost Communication With ECM/PCM A — the vehicle network has failed to receive/respond to expected messages from the engine control module. Check wiring, power/ground, and CAN bus integrity.

🔴 Repair difficulty: Hard

⏱️ Diagnostic time: 1.5-4 hours

Similar codes

Your experience will help others

+100 karma for a short comment :)

Was this AI description helpful?

Your feedback helps improve AI descriptions.

👍 Like 0 👎 Dislike 0

Send to email

U0100 — Lost Communication with ECM/PCM A