U0100 Code: Lost Communication With The ECM — Causes & Fix

U0100 means that one of your car’s other computers, usually the transmission, ABS, body or instrument cluster module, has stopped hearing from the engine computer on the vehicle’s data network. The code is stored by the module that noticed the silence, not by the engine computer itself, and that is the key to diagnosing it. U0100 is often blamed on a failed engine computer, but a failed computer is one of the least likely causes. This article covers what sets U0100, the six real causes in order of likelihood, the one question that splits them into two very different problems, and what each repair typically costs.

Start with the table, then read the triage note under it.

Cause	Telltale Sign	Typical Cost
Low or unstable battery voltage	Slow cranking, a recent flat battery or jump start, U-codes stored in several modules at once	$0 to $350
Lost power or ground to the engine computer	Engine cranks but will not start, or will not crank; a scan tool cannot talk to the engine computer at all	$10 to $250
Damaged network wiring or connector	Code comes and goes with bumps, rain or engine movement; wrong resistance across the network pins	$100 to $600
Another module or device disrupting the network	Started after a remote starter, tracker or other accessory was fitted, or after a module was replaced	$0 to $400
Software fault or mis-reported code	Stored only in one module, no drivability symptom, a service bulletin covers your vehicle	$0 to $200
Failed engine computer	Power, grounds and network all test good and the engine computer still does not respond	$500 to $1,500

Stop driving if U0100 came with the engine stalling or cutting out, the transmission stuck in one gear, or several warning lights (ABS, stability control, airbag) appearing together. Those mean the engine computer is dropping off the network while you drive, and the next dropout could shut the engine off in traffic. If the engine starts, runs and shifts normally and U0100 is only stored as a history code, the car is safe to drive to a diagnosis.

What U0100 Actually Means

Modern cars run a dozen or more control modules that share information over a two-wire data network, usually a CAN bus. The engine computer (called the ECM or PCM depending on the maker) broadcasts engine speed, throttle position, coolant temperature and torque figures many times a second. The transmission module needs those messages to decide when to shift, the ABS module needs them for traction control, and the instrument cluster needs them to move the tachometer and temperature needle.

Every module that listens for those messages also checks that they keep arriving. When the engine computer’s messages stop for longer than the listening module allows, that module stores U0100, “Lost Communication With ECM/PCM A”. The “U” means it is a network code, not a powertrain fault, and it says nothing about why the messages stopped.

The Question That Splits the Diagnosis in Two

Because U0100 is a report from a witness, the first job is to work out what the witness saw. One question does most of that work: does the engine run normally?

If the engine starts and runs normally, the engine computer is alive. It is reading its sensors, firing the injectors and running the ignition. So the fault is either in the path between it and the module that complained, or it was a brief dropout that has already passed, typically during cranking when battery voltage sags. A stored history code with no current symptom points this way, and so does U0100 in just one module.

If the engine cranks but will not start, or will not crank at all, and U0100 is current in several modules, the engine computer is probably not running. The first suspects are then its power feeds, fuses, relay and ground connections, not the network and not the computer itself. A scan tool that cannot connect to the engine computer while it talks to every other module confirms this.

The manufacturers’ own bulletins show why this matters. FCA’s service bulletin 08-068-16 for the 2016 Dodge Durango and Jeep Grand Cherokee describes U0100, along with U0140, U0101 and U0402, being stored in the ABS module as “mismapped” codes pointing to the wrong modules, with no check engine light. The fix was an ABS software update, issued, in the bulletin’s words, “to reduce the potential for misdiagnosis of related systems and incorrect module replacement.” General Motors issued similar guidance for the 2019 Cadillac CT6 in preliminary information PIC6342A. It says U0100 “may set incorrectly” in the active safety modules for a loss of communication with the transfer case module, not the engine computer, and it tells technicians to verify communication “to prevent unneeded hardware replacement.”

What the Scan Tool Can Tell You

Whether a basic code reader shows U0100 at all depends on which module stored it. Readers that use only the generic emissions interface see codes from modules that report through it. The engine computer and, on most vehicles, the transmission module do. ABS, body and instrument cluster modules usually do not. A full-system scan tool that reads every module is the right tool here. It shows which modules stored U0100, whether each code is current or history, and, on many makes, an occurrence counter or the mileage at which the code last set.

Those details narrow things quickly. U0100 in every module on the network at once means the engine computer really went silent. U0100 in a single module, with the others content, points to that module’s own wiring or software. A counter of one, set months ago, describes a single event such as a flat battery. A counter in the dozens describes a live intermittent fault. Write all of it down before clearing anything, because clearing erases the most useful evidence you have.

Symptoms You May Notice Alongside the Code

The symptoms depend on how long the engine computer was off the network and which modules missed it. A brief dropout during cranking may leave nothing but a stored code. A longer or repeated one can cause a check engine light, a transmission that holds one gear or shifts harshly (the transmission module falls back to a default mode without engine data), ABS and stability control lights, a tachometer or temperature gauge that drops to zero, a crank-no-start, or a stall that restarts after the key is cycled. When several unrelated warning lights come on together, the shared network or the power supply is a better suspect than several separate faults.

1. Low or Unstable Battery Voltage

The most common cause, and the cheapest to rule out. Control modules have a minimum operating voltage, and when battery voltage sags far enough, typically during cranking with a weak battery, some modules reset or stop transmitting for a moment while others keep listening. The listeners store U0100 and often a batch of other lost-communication codes at the same time. That is why a car that has recently had a flat battery, a jump start or a battery change so often shows a screenful of U-codes with nothing actually wrong.

Test the battery properly with a load or conductance tester, not just a voltmeter at rest. Check the terminals and the main ground cable for corrosion and tightness, and check charging voltage with the engine running. If the car keeps going flat overnight, the site’s guide to finding a parasitic battery drain covers the next step, and P0562 is the code the engine computer stores when it sees system voltage low.

2. Lost Power or Ground to the Engine Computer

The engine computer needs constant battery power, switched ignition power and a set of good ground connections. Lose any of them and it stops running, and therefore stops talking. The usual culprits are a blown fuse, a failed main relay (often labelled EFI, ECM or PCM relay in the fuse box), a corroded engine-to-body ground strap, or a loose ground bolt on the engine block or inner wing.

This cause usually shows up as a no-start with U0100 in several modules. A partial loss, such as a ground connection that breaks contact with engine movement, can instead cause intermittent stalling that clears when the key is cycled. The site’s article on a car that shuts off while driving but starts back up covers that pattern. Check each engine computer fuse with a test light or meter rather than by eye. Then, with a wiring diagram, confirm battery voltage on the computer’s power pins and a near-zero voltage drop across its grounds with the key on.

3. Damaged Network Wiring or Connector

The CAN bus is a twisted pair of wires, CAN high and CAN low, running between modules. Chafing against a bracket, a pinched harness, water in a connector, or a pin pushed back in its socket can interrupt it. A short between the two wires, or from either one to ground or power, can take down the whole network rather than one link.

There is a simple first test. A high-speed CAN bus has a 120-ohm terminating resistor at each end, and the two in parallel read about 60 ohms across the network. With the battery disconnected, measure the resistance between pins 6 and 14 of the diagnostic connector under the dash, which carry CAN high and CAN low on most vehicles. About 60 ohms is normal. About 120 ohms means one terminating resistor or one end of the network is disconnected. A reading near zero means the two wires are shorted together. From there the harness is traced section by section, starting with the engine computer connector.

Wiring faults are often intermittent. GM’s preliminary information PIT5369 for the 2015 Chevrolet City Express describes an intermittent crank-no-start with U0100 and U1000 set in multiple modules, where cycling the key made the problem disappear. The bulletin told technicians not to replace any components while engineering investigated. The later customer satisfaction program for that van, 15169B, revised a wire splice procedure.

4. Another Module or Device Disrupting the Network

Every module on the bus shares the same two wires, so one faulty module can drag the network voltage out of range and silence everyone. Aftermarket devices are frequent offenders. Remote starters and alarm interfaces tap into the network, and so do insurance trackers, fleet telematics units and cheap Bluetooth readers left plugged into the diagnostic port. A module that has taken on water, such as one under a leaking sunroof drain or a footwell, can do the same.

If the code started after anything was installed or replaced, undo that first. Otherwise, the standard technique is to watch the network (or the 60-ohm reading) while unplugging modules one at a time. When the network recovers, the last module unplugged is the problem.

5. Software Fault or Mis-Reported Code

As the FCA and GM bulletins above show, a module’s software can store U0100 when the engine computer is fine, either by mislabelling a different module’s silence or by being too strict about message timing. The signature is U0100 stored in one specific module, repeatedly, with no drivability symptom and every module answering a scan tool normally. Ask a dealer to check for service bulletins and software updates for every module involved, not only the engine computer. Many such updates are free under warranty or an emissions warranty.

6. Failed Engine Computer

It does happen, from water intrusion, heat, a voltage spike during jump-starting or welding, or internal failure. But it is a conclusion reached last, after the computer has been shown to have good power, good grounds and a healthy network connection and still does not respond. A replacement engine computer normally has to be programmed to the vehicle and often matched to the immobiliser, so a used unit from a scrapyard rarely works as a plug-in swap.

Diagnostic Order

1. Run a full-system scan and record everything: which modules stored U0100, whether current or history, occurrence counts, and every other code.
2. Test the battery, terminals and charging system. Fix any voltage problem, clear the codes, and see whether U0100 returns.
3. If the engine will not start, check the engine computer fuses, main relay, power feeds and grounds before anything else.
4. Measure network resistance at the diagnostic connector (pins 6 and 14, battery disconnected) and inspect the engine computer connector for corrosion or pushed-back pins.
5. Remove recently added accessories and unplug modules one at a time if the network is disrupted.
6. Check for service bulletins and software updates for every module that stored the code.
7. Only then condemn the engine computer.

What not to do: do not replace the engine computer, transmission module or ABS module on the strength of U0100 alone. The code comes from a witness, and both of the bulletins above were written to stop exactly that.

Repair Cost

Typical costs, labelled as typical because labour rates and access vary widely:

• Network diagnosis: about $100 to $250. Budget for it, because U0100 is a code where guessing is expensive.
• Battery, terminals or ground cable: nothing for cleaning, up to roughly $350 for a battery on vehicles with stop-start or awkward access.
• Fuse or relay: $10 to $50 in parts, plus finding out why the fuse blew.
• Network wiring or connector repair: roughly $100 to $600, depending on where the damage is.
• Software update: often free under a bulletin or warranty, otherwise up to about $200.
• Engine computer replacement and programming: roughly $500 to $1,500.

Can You Keep Driving With a U0100 Code?

It depends entirely on what the car is doing. A stored history code with no symptoms, particularly after a battery event, is not urgent. Clear it, and if it stays away, the cause was almost certainly the low-voltage dropout. If it keeps coming back without symptoms, diagnose it soon, because intermittent network faults tend to get worse rather than better.

A current U0100 with symptoms is different. If the engine computer is dropping off the network while you drive, the transmission may go into a limp mode, ABS and stability control may switch off, and the engine itself may stall if the cause is its power or ground. Have it towed or driven the shortest possible distance to a workshop.

Related Codes

• U0101: lost communication with the transmission control module. Same logic as U0100, with the transmission module as the silent party. U0100 and U0101 together point to a shared cause such as low voltage or a network fault. See the U0101 guide.
• U0121, U0140 and U0155: lost communication with the ABS module, body control module and instrument cluster respectively. Many of these at once means look at the network or battery, not at every module.
• U0001 and U0073: the high-speed network itself is faulty or a module has gone “bus off”, rather than one specific module going quiet.
• P0700: the engine computer’s flag that the transmission module has a stored fault. When U0100 is stored in the transmission module, P0700 often appears too. See the P0700 guide, and the signs of a bad transmission control module for when the transmission module itself is at fault.
• P0562: system voltage low, stored by the engine computer. Together with U0100 it makes low voltage the lead suspect. See the P0562 guide.

Frequently Asked Questions

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