U0100🔧 Vehicle network (CAN bus) and engine computerStop and fix promptly

U0100: Lost Communication With ECM/PCM "A" (Meaning, Causes, Fixes)

Reviewed 2026-10-22 · Based on 2 cited source(s)

What U0100 means

Modern cars have many computers (modules) that talk to each other over a shared network called the controller area network (CAN bus). U0100 sets when the signals between the engine control module (ECM) or powertrain control module (PCM) and another module are lost, or when the PCM can't send or receive messages on the network. OBD-Codes.com calls it a generic network code that applies to 1996 and newer vehicles, with troubleshooting that varies by model. RepairPal explains that the CAN bus uses a twisted pair of wires with a terminating resistor at each end, and that a technician should read about 60 ohms across the two network pins at the diagnostic port. If other modules are also reporting lost communication, RepairPal says the problem is likely the network itself rather than just the PCM. The sources differ on severity: RepairPal rates it Moderate, while OBD-Codes.com says the vehicle can stall and not restart until communication returns. Related codes include U0101 (lost communication with the transmission computer) and U0073 (communication bus A off).

Severity: can I keep driving?

Stop and fix promptly

Get it checked before relying on the car. The sources disagree on how serious it is: RepairPal rates U0100 Moderate, meaning it may not threaten safety right away, while OBD-Codes.com calls it a serious situation and says the vehicle can shut off at any time and not restart while communication is lost. If the engine has stalled or won't start, don't drive it, since stalling in traffic is a real safety risk; have it towed if needed.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. A faulty ECM or PCM (OBD-Codes.com, RepairPal)
  2. A faulty transmission control module (OBD-Codes.com)
  3. A problem with the CAN bus wiring or connectors, such as a short, open, or bent or pushed-out pin (OBD-Codes.com, RepairPal)
  4. A problem with the PCM's power or ground circuit, including a loose ground (OBD-Codes.com, RepairPal)
  5. A weak or dead battery (RepairPal)
  6. An aftermarket performance chip or tuner that isn't compatible with the ECM or network (OBD-Codes.com)
  7. Jump-starting with reversed polarity, which OBD-Codes.com says can destroy the computer
  8. The sources differ on emphasis: OBD-Codes.com says in its author's experience the most likely failure is the ECM, PCM or transmission module, while RepairPal says to check the battery, wiring and network before condemning the PCM

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Clear the code and see if it comes back. RepairPal says it can appear intermittently or after a dead battery.
  2. Make sure the battery is fully charged and its terminals are clean and tight, since the PCM needs a proper power supply (RepairPal).
  3. Look for broken wires, loose connectors and loose grounds at the PCM, and check for bent or pushed-out pins (OBD-Codes.com, RepairPal).
  4. If an aftermarket tuner or monitor is plugged in or installed, remove it and see if the code stops (OBD-Codes.com).
  5. Note any other U codes. Several lost communication codes at once point to the network rather than just the PCM (RepairPal).
  6. Check for technical service bulletins (TSBs) and recalls that mention U0100 for your vehicle (OBD-Codes.com, RepairPal).

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The car stalls or won't start
  • A scan tool can't communicate with the PCM at all
  • Several modules are reporting lost communication, which calls for network testing with a multimeter or breakout box (RepairPal)
  • The PCM needs replacement and programming, which needs dealer-level tools; OBD-Codes.com suggests towing to a dealer or an experienced Master ASE technician

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is U0100 serious?

It can be. OBD-Codes.com says the car can shut off and not restart while communication is lost, though RepairPal rates it Moderate. If the car has stalled or won't start, don't drive it.

Can a bad battery cause U0100?

It can. RepairPal says U0100 can result from a dead battery and that the battery should be fully charged before further testing.

Do I need a new engine computer for U0100?

Not necessarily. RepairPal says to check the battery, wiring, grounds and network before condemning the PCM. OBD-Codes.com says a failed computer is common, and a replacement usually needs programming.

How much does it cost to fix U0100?

Neither source gives a repair cost. It can range from charging a battery or fixing a connector to replacing and programming the PCM, so get a diagnosis or a written estimate first.

Sources

  1. OBD-Codes.com: U0100 Lost Communication With ECM/PCM "A" (Don Bowman, ASE Certified)
  2. RepairPal: U0100 Lost Communication With ECM/PCM "A" (rated Moderate)

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.