Network and modules
U0100Lost communication with engine control module
Lost Communication With ECM/PCM A
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Generic OBD-II code
Engine/Powertrain Control Module (ECM/PCM)
What this part does
The primary control module that manages engine (and, on combined units, transmission) operation using sensor inputs and actuator outputs.
What the computer is measuring
The modules in a modern vehicle do not each run their own wiring to everything they need. They talk to each other over a shared network, usually CAN, which is a twisted pair of wires running the length of the car. The engine computer, the transmission computer, the instrument cluster, the braking module and everything else all listen on it.
Each module broadcasts messages on a schedule, and the others expect to hear from it. When a module stops hearing the engine computer for long enough, it records U0100. So the first important point is that this code is a report by some other module about the engine computer, not a code the engine computer set about itself.
The second point follows from it. Silence is not proof of a dead computer. A module goes quiet when it loses power, when it loses its ground, when the wires carrying its messages are damaged, or when the supply voltage sags far enough that it resets. Every one of those is cheaper and more likely than a failed computer, and a weak battery alone accounts for a surprising number of these.
That is also why U0100 usually arrives with company. When the network loses the engine computer, every module that was relying on its data flags its own fault, so you end up with a scatter of codes across the vehicle. The pattern is useful: what is missing, and what is still talking, tells you where the break is.
What a generic adapter can and cannot see here
U codes are network codes, and generic OBD-II covers only part of that ground. A generic adapter, DriveDecode included, reads codes reported through the standard powertrain interface. It cannot poll every individual module on the vehicle network or run the module by module communication tests that make this code quick to trace. Knowing the code is stored is useful, and the battery, ground and fuse checks above need no special equipment at all, but a full network diagnosis is scan tool territory.
What you will notice
- Check engine light on, frequently with several other warning lights at once.
- Multiple unrelated codes stored across different modules.
- Gauges reading zero or behaving erratically.
- A no start, or an engine that starts and stalls immediately.
- The vehicle running in a reduced power mode.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
A weak battery or a poor battery connection
The most common cause and the one people skip. Low or unstable voltage drops modules off the network, and it produces a scattering of communication codes across the whole vehicle.
A corroded or loose ground connection
Second most common. Engine to body and engine to battery grounds corrode, and a module that cannot find a solid ground cannot communicate reliably.
Damaged network wiring
The vehicle network is a twisted pair running the length of the car. A chafed wire, a short between the two lines or a break stops traffic for everything downstream of it.
A blown fuse feeding the engine computer
A computer with no power says nothing, and every other module reports it as missing.
Water intrusion into a connector or module
A common cause after flooding, a leaking screen seal or a windscreen leak, particularly where modules sit low in the footwell.
A failed engine control module
Genuinely last on this list. It is expensive, often needs programming, and it is the wrong first conclusion the great majority of the time.
What to check, in order
Test the battery and load test it, and clean and tighten both terminals.
A weak battery causes a remarkable share of communication codes. Codes that do not return after a good battery and clean terminals were caused by voltage, not by a module.
Check the main ground straps between the engine, the body and the battery.
A corroded or loose ground explains erratic communication across multiple modules at once. Clean, tight grounds rule that out.
Check the fuses and relays feeding the engine control module.
No power or no ground at the module explains everything downstream, and it is a wiring repair rather than a module repair.
Note exactly which modules are reporting lost communication and which are not.
The pattern locates the break. A single module missing points at that module or its own wiring. Everything missing at once points at power, ground or the main network trunk.
Inspect the network wiring for chafe points, and look for water intrusion at low mounted connectors.
Damage found here is the fault. Only when power, ground and wiring are all proven good does the module itself become a reasonable suspect.
Where this one turns up most
- Vehicles with an ageing battery, particularly in cold weather.
- Vehicles that have been jump started incorrectly or had recent electrical work.
- Anything with a history of water leaks into the footwell or the boot.
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading U0100 on your own car
DriveDecode pulls U0100 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-05. Definition sourced from the published standard.