# U0100 code: Lost communication with ECM/PCM A

Source: https://codeandclear.com/blog/u0100-code
Published: 2026-10-09
Publisher: Code & Clear

**Short answer:** U0100 means another module on the vehicle network, such as the ABS, transmission or instrument cluster, stopped hearing messages from the engine computer (ECM or PCM). Common causes are low battery voltage, a blown fuse or bad ground for the engine computer, and damaged or corroded network wiring. It can cause no-starts or stalling, so diagnose it promptly.

This page is for drivers and DIY mechanics who found U0100, often along with several warning lights at once. U codes are network codes: they describe a communication problem between modules, not a failed sensor. That makes them feel intimidating, but most causes are basic electrical faults you can check with a multimeter. We explain the code, the symptoms, causes from simplest to most involved, a diagnosis plan, and whether it is safe to drive.

## What U0100 means

U0100 is a generic SAE J2012 network code: **Lost Communication With ECM/PCM "A"**. The U stands for network (communication between modules), and the 0 means the definition is generic. ECM is the engine control module and PCM is the powertrain control module; "A" refers to the main one.

Modules on modern vehicles share information over a network, usually CAN bus. Each module expects regular messages from the engine computer. When a module goes too long without hearing from it, that module stores U0100. So U0100 is usually stored in another module, like the ABS, transmission, body or cluster, and you may need a scan tool that reads those modules to see it. Learn more about how codes are built with our [code decoder](https://codeandclear.com/tools/code-decoder).

## Symptoms of U0100

Symptoms depend on whether the loss of communication is constant or intermittent. A one-time drop during a weak battery start may leave only a stored code.

- Check engine light and often several other warning lights at once (ABS, traction, airbag)
- Engine cranks but does not start, or no communication with a scan tool on the engine
- Stalling or intermittent shutoff while driving
- Gauges dropping to zero or acting erratically
- Transmission in limp mode or harsh shifts
- Code may be stored as history even when the car runs fine now

## Common causes of U0100

A low battery is a surprisingly common trigger. Modules can drop off the network during cranking when voltage dips, then store U codes.

| Cause | How to check | Typical fix |
| --- | --- | --- |
| Weak battery or poor battery terminals | Test battery voltage and condition; inspect and clean terminals | Charge or replace battery; clean and tighten terminals |
| Blown fuse or failed relay feeding the engine computer | Check all fuses and relays listed for the ECM/PCM | Replace fuse or relay and find the cause |
| Bad ground for the engine computer | Voltage drop test the ground straps and module grounds | Clean or repair ground connections |
| Damaged, corroded or water-soaked network wiring or connectors | Inspect harness and module connectors; check CAN resistance at the OBD2 port with battery disconnected | Repair wiring or connector |
| Aftermarket device on the network (remote start, tracker, tuner) | Unplug the device and recheck | Remove or reinstall the device correctly |
| Failed engine computer | Only after power, ground and network wiring test good | Replace and program per service information |

U0100 causes, checks and fixes

## How to diagnose U0100

If the car cranks but will not start and no tool can talk to the engine computer, [our no-link guide](https://codeandclear.com/blog/obd2-scanner-no-link-fix) covers the scan tool side of the problem.

1. Scan all modules, not just the engine, and note which modules stored U0100 and any other U codes. A full-system scan tool helps here; see the [best OBD2 scanner for ABS and SRS](https://codeandclear.com/best/best-obd2-scanner-for-abs-and-srs) guide.
2. Check whether the scan tool can talk to the engine computer at all. If it cannot, focus on power, ground and wiring at the engine computer.
3. Test the battery and charging system. Clean and tighten battery terminals and main ground straps.
4. Check every fuse and relay that feeds the engine computer, using the service information for your vehicle.
5. Inspect the engine computer connector and network wiring for corrosion, water, pushed-back pins and chafing.
6. With the battery disconnected, measure resistance between CAN high and CAN low at the OBD2 port (pins 6 and 14). Many CAN systems read about 60 ohms with both terminating resistors in place; a very different reading points to a wiring or module problem.
7. Unplug any aftermarket devices and retest.
8. Only after power, ground and wiring check out, suspect the engine computer itself. A replacement usually needs programming.

## How to fix it and can you drive with U0100?

Fix the fault the checks point to: battery, terminals, fuse, ground or wiring. These cover many U0100 cases. Replacement engine computers usually need to be programmed to the vehicle, which often means a dealer, a shop, or a [scan tool with ECU coding](https://codeandclear.com/best/best-scan-tool-with-ecu-coding) that supports your make.

Do not treat U0100 lightly if it comes with symptoms. If the engine stalls, the transmission goes into limp mode or several warning lights come on while driving, stop driving and have the car towed or checked. If the code is only stored as history after a weak-battery start and the car runs normally, fix the battery and recheck.

Note: Disconnect the battery before unplugging modules or probing connectors as the service information directs. Airbag (SRS) wiring is often nearby; never probe yellow SRS connectors and follow the airbag safety steps in your service information.

## Related codes

[U0101](https://codeandclear.com/blog/u0101-code) is lost communication with the transmission computer, and [U0001](https://codeandclear.com/blog/u0001-code) is a high-speed CAN bus fault. An internal engine computer fault such as [P0606](https://codeandclear.com/blog/p0606-code) can appear with U0100. Several warning lights together may also show up as an [ABS light](https://codeandclear.com/blog/abs-light-on-causes) or [airbag light](https://codeandclear.com/blog/airbag-light-on).

## Frequently asked questions

**Does U0100 mean my ECM is bad?** Not necessarily. Low battery voltage, a blown fuse, a bad ground or network wiring faults are more common. Check those before replacing the engine computer.

**Can a bad battery cause U0100?** Yes. Low voltage during cranking can make modules lose communication and store U codes. Test and charge or replace the battery first.

**Why can my scanner not read U0100?** U0100 is usually stored in other modules, not the engine computer. Basic code readers often only read engine codes. You need a tool that scans all modules.

**Can I clear U0100 and keep driving?** If the cause was a one-time low-voltage event and the car runs normally, clearing it may be fine. If it returns or comes with stalling or warning lights, have it diagnosed before driving further.

## Gear guides for this topic

- https://codeandclear.com/best/best-obd2-scanner-for-abs-and-srs
- https://codeandclear.com/best/best-scan-tool-with-ecu-coding
- https://codeandclear.com/best/best-bidirectional-scan-tool
- https://codeandclear.com/best/best-obd2-scanner-for-diy-mechanics