If your car is showing the U0100 engine code, it means that your car has lost communication with the engine control module (ECM) and/or the powertrain control module (PCM), which can cost $800 to $1,500 to repair.

A U0100 engine fault code (“Lost Communication with ECM/PCM”) can cause your engine to run hot, shut down, or not start altogether.
Fixing DTC U0100 requires a diagnosis from a repair shop, with fixes running from very minor adjustments to full ECM or PCM replacements running $800 to $1,500.
Common symptoms of this error code include a check engine warning light, a drastic drop in fuel economy, trouble starting your car, and even engine failure.

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Engine code U100: “Lost communication with ECM/PCM”

Diagnostic trouble code U0100 is a generic network communication code that means your car’s high-speed controller area network bus (CAN C) can’t receive messages from the engine control module (ECM) and/or the powertrain control module (PCM).

These electronic control units (ECU) serve the following purposes:

ECM: Gathers information about how the engine is functioning, provides the right amount of fuel to the engine, and minimizes emissions.

PCM: Works closely with the ECM to regulate fuel-to-air ratios in your engine.

CAN C: Connect all the other data-collecting devices to your car’s PCM to conduct your car’s mechanics in tandem.

Serious risk: If the line of communication to the car’s CAN-C is lost, your car’s data bus won’t be able to regulate the engine properly. To avoid straining your engine, get a U0100 code checked out by a mechanic as soon as possible.

Costs for fixing engine code U0100 vary greatly

The cost to fix problems associated with DTC U0100 varies greatly depending on the nature of the communication loss and the make of your vehicle.

What to expect: A mechanic should only take one hour to troubleshoot and diagnose the cause of your U0100 code with one of the following average estimates:

Problem with battery or connector wiring between the CAN bus and the PCM (quick fix): cost of labor

ECM or PCM replacement: $800 to $1,500

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Possible causes of the U0100 engine code

The U0100 code shows up when there is a loss of communication to the ECM or the PCM. The most common causes for this include:

Loose connections

Broken CAN C bus wiring

Damaged wiring harness

A low or malfunctioning battery

A problem with the PCM computer itself

The bottom line: The PCM acts like the brain in your vehicle, gathering information from the ECM and other modules through the CAN bus. DTC U0100 will be triggered if the CAN-C bus does not receive a periodic message from ECM/PCM to establish connection.

Common symptoms of a U0100 engine code

Your

check engine light

will turn on in the case of a U0100 engine code. Other common symptoms of a U0100 code include:

A drastic drop in fuel economy

Trouble starting your car

Increased emissions

Problems shifting

Car stalling

Engine running hot

Engine failure

You’ll need a mechanic to fix engine code U0100

In most cases, you should hire a professional mechanic to fix the U0100 engine code; however, you may be able to rule out some issues if you’ve picked up a U0100 code with your OBD II scan tool at home.

What to do: Make sure your car battery is charged and check for broken wires or damaged components inside your car. You may be able to see if wires are disconnected, but if the problem lies in the PCM itself, more complicated repairs will be required.

MORE: How to read car OBD codes

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FAQs

How do I fix a U0100 engine code?

A U0100 engine code may involve a complicated fix that’s best left to the professionals. You can check to make sure your battery is charged and that the wire connections are secure, but there’s not much else you can do unless you have training in advanced car maintenance.

Can I use a PCM from a different make of car?

Generally, PCMs are made specifically for your type of car and vehicle manufacturer. For example, buying a Honda PCM for a Ford vehicle will most likely not work and leave you with a useless and expensive PCM.

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