U0100

Code

Generic U — Network/User

Lost Communication With ECM/PCM A

Brand: Generic

Type: U — Network/User

Views: UK: 74 EN: 130 RU: 159

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Blown fuse or power supply fault to ECM/PCM
Poor or corroded battery/ground connections
Open, shorted, or damaged wiring in power, ground, or data lines (CAN/LIN)
Faulty ECM/PCM or another module asserting bus-off
Failed or missing termination resistor(s) on communication bus
Intermittent connector contact or pin damage

Symptoms

Malfunction indicator lamp (MIL) or other warning lights illuminate
No engine data or live parameters available from ECM/PCM with scan tool
Crank or no-start conditions if ECM/PCM not powered
Loss of engine-related features (cruise, transmission shifting faults) or limp-home behavior
Multiple modules reporting network-related faults or reduced functionality

What to check

Verify battery voltage and state of charge (12.4–12.8 V resting; >13.5 V with engine running)
Check all fuses and relays related to ECM/PCM power and ignition feed
Inspect main ECM/PCM connector for corrosion, bent pins, or pushed-out pins
Scan all modules to see which modules are present and which report U-codes
Check CAN bus physical layer: measure CAN_H and CAN_L voltages and termination resistance
Look for aftermarket accessories connected to data or power circuits and disconnect them

Signal parameters

CAN_H idle voltage: typically ~2.5–3.5 V (vehicle dependent)
CAN_L idle voltage: typically ~1.5–2.5 V (vehicle dependent)
Differential voltage during dominant state: commonly ~1.0–2.0 V
Bus termination resistance: approximately 60 ohms measured between CAN_H and CAN_L (two 120Ω in parallel)
Expected CAN bus traffic: presence of regular message frames at vehicle baud rate (e.g., 125 kbps, 250 kbps, 500 kbps)
Bus error counters and 'bus-off' status reported by modules

Diagnostic algorithm

Record freeze-frame data and all U-codes and related P/U codes from scan tool. Note when the fault was set (key on/off, driving, startup).
Verify battery condition and main power/ground feeds to ECM/PCM. Repair any low voltage or bad ground before further testing.
Check ECM/PCM power and constant/ignition feed fuses and relays. Replace or test relays and fuses as required.
Visually inspect ECM/PCM connector and nearby wiring for damage, corrosion, moisture, or pin issues. Repair wiring and connectors as needed.
Use a scan tool to poll the ECM/PCM. If no response, scan other network modules to determine which nodes are present and which also fail to communicate.
Measure CAN_H and CAN_L voltages at the ECM/PCM connector with key ON (engine off) and compare to expected values. Check termination resistance between CAN_H and CAN_L with ignition off.
If voltages or termination are out of range, isolate the short/open by disconnecting harness sections or peripheral modules and rechecking the bus until the fault clears or the problematic section is found.
Inspect for modules in 'bus-off' condition. If a single aftermarket or failed module is causing bus issues, disconnect it and verify network operation.
If wiring and external modules check good but ECM/PCM still unresponsive, consult vehicle-specific tech docs for ECM programming/flash or replacement procedures. Do not replace ECM/PCM unless other tests confirm module fault.
After repairs, clear codes and perform a road/test cycle and re-scan to confirm the U0100 does not return.

Likely causes

ECM/PCM lost battery power or main relay/fuse open
CAN high or CAN low shorted to battery or ground near ECM/PCM
Damaged connector or corroded pin at ECM/PCM harness
Another control module in bus-off state preventing network communications
Open or high-resistance ground at engine/chassis ground strap

Fault status

⚠️ Status

Lost Communication With ECM/PCM A — the ECM/PCM is not responding on the vehicle data bus. Verify power and network integrity before replacing the module.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-4 hours