U0100: Lost communication with ECM/PCM "A"
Another control module, such as the gearbox or ABS module, has lost contact with the engine computer over the network. The engine computer itself may be losing power.
Updated
What U0100 means
The control modules in a car talk to each other over a network, the CAN bus, and each expects regular messages from the others. When a module stops hearing one of them, it stores a “lost communication” code that names the silent module. U0100 means the engine computer (ECM or PCM) went quiet.
The code is stored by the module that noticed, not by the silent one: U0100 is often stored by the gearbox or ABS module. Read My Car reads the engine computer and, on many cars, the gearbox module over generic OBD2; codes stored in the ABS, body or instrument modules need a tool for your make.
A module that loses its power or earth also goes quiet, so these codes often come with battery or voltage problems.
U0101, U0140 and U0155: the gearbox, body module and instruments
The same kind of code, naming a different module that went quiet. What it means for driving depends on what that module does, so each has its own driving advice below.
U0101: Lost communication with TCM
Another control module, often the engine computer, has lost contact with the gearbox module (TCM) over the network. The gearbox may go into limp mode.
Common causes: loss of power or earth to the gearbox module, such as a blown fuse, network wiring or connector fault, faulty gearbox module.
Driving: Drive gently and only as far as you need to, and have it checked soon. The gearbox may shift harshly or stay in one gear (limp mode), so avoid motorways and overtaking until it’s fixed. If it slips, won’t change gear or won’t drive, don’t force it: have it recovered.
U0140: Lost communication with body control module
Another control module has lost contact with the body control module, which runs things like the lights, wipers, locks and, on many cars, the immobiliser.
Common causes: loss of power or earth to the body control module, such as a blown fuse, network wiring or connector fault, faulty body control module, sometimes after water got in.
Driving: Drive gently and have it checked soon. Lights, wipers, locks or the immobiliser may stop working, and on some cars the engine may not restart once switched off. Don’t drive in the dark or in rain if the lights or wipers don’t work.
U0155: Lost communication with instrument panel cluster (IPC) module
Another control module has lost contact with the instrument cluster, so the gauges and warning lights may freeze, go blank or be wrong.
Common causes: loss of power or earth to the instrument cluster, such as a blown fuse, network wiring or connector fault, faulty instrument cluster, sometimes after water or coolant got into the dashboard.
Driving: Drive gently and have it checked soon. The instruments may freeze or go blank, so you may not see your speed or warnings such as low oil pressure or overheating. Don’t drive far without a working speedometer and warning lights.
Can I keep driving?
Drive gently
Drive gently and only as far as you need to, and have it checked soon. Other modules may run in a fallback mode without the engine computer’s data, and if the engine computer is losing power, the engine can cut out and may not restart. If the engine stalls or several warning lights come on, don’t drive on: have it recovered.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
If U0100 comes with stalling, a no-start or many warning lights at once, the engine computer itself may be losing power: don’t drive on.
Symptoms
- Several warning lights at once.
- With U0100: the engine stalls, runs in limp mode or won’t start.
- With U0101, the gearbox stays in one gear; with U0140, lights, locks or wipers misbehave; with U0155, the gauges freeze or go blank.
Common causes
- Loss of power or earth to the engine computer, such as a blown fuse. Check the fuses and the earth connections for the module the code names first.
- Network wiring or connector fault. A damaged wire, or a corroded or loose connector, between the modules.
- Faulty engine computer. Only after its power, earth and network wiring check out.
Typical fixes
- Check the battery, its clamps and the earth straps: low voltage makes modules drop off the network.
- Check the fuses for the module the code names.
- Clear the codes and read them again after a drive: one stored during a flat battery or a jump start may not come back.
- Have the module’s power, earth and network wiring checked. Replace the module, and have it programmed to the car, only if those are fine.
Questions
Can I drive with U0100?
Gently and only as far as you need to, and have it checked soon. If the engine computer is losing power, the engine can cut out and may not restart. If the engine stalls or several warning lights come on, don’t drive on: have it recovered.
Which module stored U0100?
Usually not the engine computer itself, but a module that stopped hearing it, such as the gearbox or ABS module. Read My Car shows which control unit reported each code, where the car shares it over generic OBD2.
Can a weak battery cause U codes?
Yes. Modules drop off the network when the voltage is too low, so a weak battery or a poor connection can set lost-communication codes in several modules. Check the battery first.