Car & driving · Free
U0100: lost communication with the engine control module (ECM/PCM)
U0100 means another control unit in the car could no longer hear the engine computer on the data network. The causes range from a weak battery and corroded plugs to a broken bus wire or a failed engine computer. Start with the voltage.
- No account
- Runs on your device
- Phone & desktop
Check your car for U0100
Runs on your device
Connect your OBD2 adapter
- Plug the adapter into the OBD2 port – usually under the steering wheel, near the pedals.
- Switch the ignition on, or start the engine.
- Bluetooth adapter? Turn Bluetooth on. Nothing else to set up.
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What U0100 means
Modern cars connect their control units through a data network, usually the CAN bus. The engine computer sends messages on it all the time. If the gearbox, ABS or dashboard unit stops receiving them, that unit stores U0100: lost communication with the engine control module.
Because the engine computer may be fine and simply unreachable, U0100 is often stored in several other units at once. Standard OBD2 reads the engine and gearbox units, so you see U0100 when the gearbox unit has stored it; codes in ABS or body units need a manufacturer-level tool.
Symptoms of U0100
- The engine does not start or stalls while driving.
- Several warning lights at once and gauges that drop to zero.
- The gearbox in limp mode.
- Sometimes nothing at all, if the dropout was short.
Causes of U0100, cheapest first
Work down the list in order: the first checks cost little or nothing, the last ones are jobs for a workshop.
Low battery voltage
A weak battery or bad terminals make control units reset and drop off the bus. Check the voltage first.
A blown fuse or faulty relay
The engine computer and the network have their own fuses and a main relay. A failed one cuts the computer off.
Corroded or loose connectors
Water in plugs, especially at the engine computer and under the carpets, causes dropouts that come and go. Look for green corrosion.
Damaged bus wiring
Chafed or broken CAN wires, often near hinges, the engine or old accident repairs. A workshop measures the bus resistance at the OBD port.
Aftermarket devices
Trackers, alarms and cheap adapters left plugged in can disturb the bus. Unplug them and see whether the fault disappears.
A failed engine computer
The expensive end of the list, to consider only after power, ground and wiring are proven good.
Can you drive with U0100?
Do not rely on the car until it is checked. If U0100 comes with stalling or a no-start, the car can fail at any moment. Avoid driving until the cause is found. If it appeared once after a battery change or a jump start, clear it and watch whether it returns.
Check U0100 with OBD Studio
Connect your adapter above, or try the demo car, then:
- Read Control module voltage in Live data with the engine off and running. Low or unstable voltage is the most common trigger.
- In Vehicle, look at Control units that answered: if the engine unit does not answer at all, the problem is its power, ground or bus connection.
- Check the earlier scans in Fault codes: a U0100 that appeared once after a jump start is less worrying than one that keeps returning.
- After a repair, clear the codes and run a few scans to see that every control unit keeps answering.
More fault code guides
Each of these codes has its own page with causes ranked by cost and the checks that go with them.
- P0420Catalyst efficiency below threshold
- P0171System too lean
- P0300Random or multiple misfire
- P0455EVAP system large leak
- P0442EVAP system small leak
- P0128Engine warms up too slowly
- P0172System too rich
- P0401EGR flow too low
- P0101Air flow sensor implausible
- P0135Front O2 sensor heater
- P0016Crankshaft and camshaft out of step
- P0011Intake camshaft timing over-advanced
- P0141Rear O2 sensor heater
- P0113Intake air temperature sensor high
- P0457Fuel cap loose or missing
- P0299Turbo underboost
- P0700Transmission control system fault
- P0087Fuel rail pressure too low
- P0562System voltage low
More OBD Studio pages
Frequently asked questions
Can a bad battery cause U0100?
Yes. Low or unstable voltage can make the engine computer reset or drop off the network. Check the battery and its terminals before you suspect the wiring or the computer.
Will U0100 clear itself?
If the dropout happened once, for example during a jump start, the code stays stored until it is cleared but does no harm. If it keeps returning, there is a real power or wiring fault.
Why can a scanner sometimes not connect when U0100 is stored?
If the engine computer is really offline, it cannot answer the scanner either. When OBD Studio finds no engine at all, check the fuses and the voltage at the engine computer.
Can I read U0100 without going to a workshop?
Yes. With any ELM327-compatible Bluetooth LE or USB adapter, OBD Studio on this page reads U0100 and every other stored code for free, shows the freeze frame and clears the code after the repair. Nothing to install, no account.
