U0100: Lost Communication With Engine Control Module A
Do not keep driving
The car will most likely not start, or will have stopped, with a dashboard either dead or lit up with warnings from every system at once.
The most useful thing to know is that this code does not mean the engine computer has failed. It means it has stopped being heard on the network the vehicle's computers share, and there are several commoner reasons for that than a dead module — its power supply, its earth connection, the two bus wires themselves, or another module elsewhere jamming the network so nothing can be heard.
That distinction is worth money. Replacement engine modules are expensive and normally have to be programmed to the vehicle, which is labour you cannot get back if it turns out the module was fine. Ask what has been checked before agreeing to one: the module's power and earth, the bus wiring, and whether other modules are also reporting lost communication.
If a lot of unrelated codes appeared at once, say so — that pattern usually means a single fault on the shared network rather than many things failing together, and it points at a very different and much cheaper investigation.
Mention any recent electrical work, an aftermarket alarm or tracker, a jump start, or water getting into the car. All four are ordinary causes of network faults.
What this code means
The engine control module has stopped being heard from on the vehicle's data network.
This is the first code in the corpus from the network category, and the single most important thing to understand about all of them is in the title: lost communication, not failed module.
A modern vehicle is a dozen or more computers sharing a pair of twisted wires. Rather than run a separate wire for every value, each module broadcasts what it knows on a schedule and every other module listens for what it needs. Engine speed, road speed, throttle position, coolant temperature, torque, gear — all of it travels as messages on a shared bus. A lost communication code means the expected messages stopped arriving. It says nothing about why.
The causes, in the order they are actually found:
- Power or earth to the named module. A module with no supply cannot speak, and reports exactly as one that has died. This is the commonest real cause and the cheapest, and it is the corpus's standing argument about module faults — a module short of voltage misbehaves rather than announcing why.
- The bus wiring itself. Two wires, twisted deliberately so that interference affects both equally, plus the connectors and splices along them. One chafed conductor, one corroded pin, one splice full of water.
- Another module dragging the bus down. This is the one worth knowing about. A single failed module can hold the bus in a state where nobody can talk — so every module reports lost communication with every other module. A scan tool showing a dozen lost-communication codes across the whole vehicle is describing one fault, not a dozen, and the fastest route is usually to disconnect modules one at a time until the bus recovers.
- A gateway module, on vehicles where separate buses are bridged. Everything on the far side goes quiet at once, which looks catastrophic and is one part.
- The named module itself. Last, not first.
Why the ordering matters more here than almost anywhere else in the corpus: the code names a module, so the module gets replaced. A new module on a broken bus does not talk either — and on most vehicles a replacement has to be programmed and coded to the vehicle before it will work at all, which is labour that cannot be refunded with the part. A wrongly-replaced module costs considerably more than its price.
One diagnostic distinction is worth having before anyone starts. A scan tool that cannot connect to the vehicle at all is a different situation from a stored U0100 read from another module. In the second case the bus is working well enough for the tool to be having a conversation on it — so the wiring between the socket and the network is proven, and the problem is narrower than it looks.
With the engine module specifically, everything downstream loses its reference at once: the transmission has no torque figure, the dashboard has no engine speed, and the vehicle will usually not start or will stop. That breadth is a consequence of what stopped talking rather than evidence that the engine is damaged.
Likely causes
Start here
- Failed power or earth supply to the module · moderate
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Another module holding the bus down · specialist
- Blown fuse or failed supply · easy
Less common
- Faulty aftermarket alarm or immobiliser installation · moderate
- Water ingress into connector · moderate
- Faulty engine control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about U0100.