U0100: Lost Communication With ECM/PCM 'A'

Quick Answer

• What it means: The vehicle's control modules have lost communication with the Engine Control Module (ECM) or Powertrain Control Module (PCM), which is the "brain" of the vehicle.
• Severity: Get towed immediately. This is a severe issue that can lead to a non-starting vehicle or sudden loss of power while driving.
• Most common cause: A faulty ECM/PCM or an issue with the Controller Area Network (CAN) bus communication circuit.

What the U0100 code means

The U0100 diagnostic trouble code (DTC) officially stands for "Lost Communication With ECM/PCM 'A'". In simpler terms, this code indicates that other control modules in your vehicle are not able to communicate with the Engine Control Module (ECM) or Powertrain Control Module (PCM). The ECM/PCM is vital for controlling engine functions, transmission shifting, and various other critical systems. When communication is lost, these systems cannot operate correctly, or at all.

Modern vehicles rely heavily on a network of interconnected control modules that communicate with each other over a data bus, most commonly the Controller Area Network (CAN) bus. This intricate network allows for the rapid exchange of information necessary for the vehicle to function. When the U0100 code is set, it means that one or more modules sending or receiving signals from the ECM/PCM are not getting a response, or the ECM/PCM itself isn't transmitting data.

Common symptoms

• Check Engine Light (CEL) illuminated: This is virtually guaranteed with a U0100 code.
• No start condition: The engine may crank but not start.
• Engine stalling: The engine may suddenly shut off while driving.
• Erratic engine operation: Unpredictable performance, rough idle, or hesitation.
• Transmission shifting problems: Harsh shifts, delayed shifts, or being stuck in one gear.
• Inoperative gauges or warning lights: Instrument cluster may not function correctly.
• Other warning lights: ABS, traction control, or airbag lights may illuminate.
• Vehicle entering "limp mode": Reduced engine power and limited vehicle speed.

Common causes

1. Faulty ECM/PCM: The ECM/PCM itself may have failed internally, preventing it from communicating with other modules.
2. CAN bus wiring issues: Damaged, corroded, or open circuits in the CAN bus wiring can disrupt communication. This can include frayed wires, loose connectors, or shorts.
3. Faulty ground connection for the ECM/PCM: A poor or corroded ground connection can prevent the ECM/PCM from powering up or communicating correctly.
4. Blown fuse: A fuse protecting the ECM/PCM or a related communication circuit may have blown.
5. Faulty gateway module (if applicable): Some vehicles use a gateway module to route communication between different CAN bus networks. A fault in this module can isolate the ECM/PCM.
6. Software corruption or programming error in the ECM/PCM: Less common, but possible, especially after a battery jump-start or programming attempt.
7. Water intrusion into the ECM/PCM: Water damage can short circuit the internal components of the module.

How a technician diagnoses it

1. Verify the complaint and perform a visual inspection: The technician will confirm the symptoms and look for obvious signs of damage, such as chewed wires, loose connectors, or corrosion around the ECM/PCM and main wiring harnesses.
2. Scan for all DTCs in all modules: A professional scan tool will be used to check for not only the U0100 code but also any other communication-related codes (U-codes) in all accessible control modules. This helps pinpoint which modules are communicating and which are not.
3. Check for power and ground to the ECM/PCM: Using a multimeter, the technician will verify that the ECM/PCM is receiving proper power and ground. Missing voltage or an open ground circuit will prevent the module from operating.
4. Inspect the CAN bus wiring: The technician will specifically check the high-speed and low-speed CAN bus wires leading to and from the ECM/PCM for continuity, shorts to ground, and shorts to voltage. This often involves checking resistance across the CAN bus lines and looking for proper voltage signals.
5. Check CAN bus termination resistors: The CAN bus requires specific termination resistors at each end of the network to ensure signal integrity. Incorrect resistance (often 60 ohms between CAN high and CAN low) can indicate an open or shorted terminating resistor.
6. Monitor live data with a scan tool: If some communication is present, the technician may try to monitor live data from the ECM/PCM to see if any signals are being transmitted or received intermittently.
7. Perform manufacturer-specific diagnostic tests: Many vehicle manufacturers have specific diagnostic routines and tests for communication issues that can be accessed with a factory-level scan tool.
8. Internal ECM/PCM diagnosis (if all external checks pass): If all wiring, power, and ground checks are good, and the termination resistors are correct, the technician may suspect an internal ECM/PCM fault. This might involve sending the ECM/PCM out for testing or replacement with a known good unit.

Typical repair cost

Repair costs for a U0100 code can vary significantly depending on the diagnosed cause and the vehicle make/model.

• Wiring repair or fuse replacement: $100 - $350 (parts and labor).
• Ground repair: $75 - $200 (parts and labor).
• ECM/PCM replacement: $800 - $2,500+ (parts and labor). This cost can be higher for luxury or specialized vehicles, as programming and calibration are also required.
• Gateway module replacement: $300 - $800 (parts and labor).

These figures are estimates and can fluctuate based on hourly labor rates and parts availability.

Can I keep driving with this code?

No, absolutely not. A U0100 code indicates a severe communication breakdown with the engine