U0101 Lost Communication With TCM
U0101 is a generic OBD2 network code. It means “Lost Communication With TCM” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker.
Ask the AI about U0101 on your car- System
- Network (U)
- Code type
- Generic (SAE)
- Range
- U01xx
- Urgency
- High
What U0101 means
The car’s modules talk to each other over a network (CAN bus). This code means the module that stored it stopped hearing the module named in the code — a symptom of a power, wiring or module problem rather than a part to replace.
Symptoms
- Several warning lamps at once
- The silent module’s functions not working — gauges, ABS, locks or the transmission, depending on the code
- Sometimes a no-start or limp mode (U0100, U0101)
Most likely causes
- Low battery voltage or a poor battery connection — enough to set U-codes across the car
- A blown fuse or failed relay feeding the silent module
- Damaged network wiring or a corroded connector — water getting in is common
- A failed module
What to check first
- Check the battery and its terminals first, then clear the codes and see which come back
- Check the fuses and the power and ground at the named module
- Inspect the network wiring and connectors near that module
Can I keep driving with U0101?
Limit driving — get it checked promptly
It depends on what is offline: lost contact with the engine, transmission, ABS or airbag module is a safety matter — get it checked straight away. Comfort modules can wait, but check the battery soon.
What the wording tells you
“Lost communication” means the module that stored the code stopped receiving messages from the module named in it. That named module is the one to look at — its fuse, power and ground, its connector and the network wiring to it — along with the battery, since a weak one can make several modules drop off at once.
How U0101 breaks down
Network. Communication between the car’s control modules.
Generic code: defined by SAE, the same on every make.
Lost communication between modules.
The specific fault within this range.
Where to look: Lost communication between modules
Codes stored when one module stops receiving messages from another — the engine or transmission computer, ABS, airbags, body control, instrument cluster or dozens of others. The code names the module that went silent, not the one that stored it.
All U01xx codes →Ask the AI about U0101 on your car
Pick your car and ask — the assistant takes your make, model and engine into account. Ten questions a day, free, no sign-up.
The assistant is answering…
That's 10 for today
The free web limit resets tomorrow. In the Brimly app the assistant knows your specific car — its real mileage, fuel log, and service history.
Requires iOS 18.1 or later. Free to download, with optional in-app purchases.
The conversation is stored only in your browser — we don't keep it on our servers.
Read and clear U0101 on your iPhone
Brimly reads stored, pending and permanent codes through a Bluetooth LE or Wi-Fi OBD2 adapter, explains each one offline from the same 2,693-code dictionary as this page, and clears them once the fault is fixed — each clear goes into the car’s service history with the codes it removed. OBD diagnostics are part of Brimly Forever, a one-time purchase; the PRO assistant explains a code for your exact car.
Requires iOS 18.1 or later. Free to download, with optional in-app purchases.
Frequently asked
What does U0101 mean?
U0101 is a generic OBD2 network code. It means “Lost Communication With TCM” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker. The car’s modules talk to each other over a network (CAN bus). This code means the module that stored it stopped hearing the module named in the code — a symptom of a power, wiring or module problem rather than a part to replace.
Can I drive with U0101?
It depends on what is offline: lost contact with the engine, transmission, ABS or airbag module is a safety matter — get it checked straight away. Comfort modules can wait, but check the battery soon.
What causes U0101?
Low battery voltage or a poor battery connection — enough to set U-codes across the car; A blown fuse or failed relay feeding the silent module; Damaged network wiring or a corroded connector — water getting in is common; A failed module.
How do I clear U0101?
Fix the cause first, then clear it with any OBD2 scanner or app. Once the fault is gone the car also clears it on its own: the check engine lamp goes off after the self-test passes on three drive cycles in a row, and the stored code ages out after about 40 warm-ups. Clearing without a repair only hides it for a while — and resets the readiness monitors an emissions inspection looks at.
Is U0101 the same on every car?
Yes. U0101 is a generic SAE J2012 code, so its definition is identical on every make and model. What differs is the cause behind it and the repair procedure, which depend on the engine — that’s why the assistant on this page asks for your car.
Guides
Definitions: the generic SAE J2012 codes from Brimly’s offline dictionary (data version 2026.09). Causes and advice are general guidance for all makes — the service information for your car has the last word.