U0100: Lost Communication With ECM/PCM A
Key takeaways
- U0100 means a module on the car's network, such as the transmission, ABS or body computer, stopped receiving messages from the engine computer.
- If the engine won't start or stalls, don't drive it; if it runs normally and the code came after a jump start or battery change, it may be a one-time glitch.
- Most U0100 fixes are cheap and electrical: a weak battery, a loose terminal, a corroded ground or a blown fuse come before the wiring and the module.
- A replacement engine computer runs $800–$1,800 with programming, so the network should be tested before one is ordered.
- 86 owner complaints filed with NHTSA mention U0100, and 165 manufacturer bulletins from 19 makes name it.
What U0100 means
Other computers in your car stopped hearing from the engine computer. Scanners call it “Lost Communication With ECM/PCM A.” Modern cars run a data network (the CAN bus) that links the engine computer (ECM or PCM) to the transmission, ABS, body, instrument cluster and others. Each module expects regular messages from the engine computer; when those messages stop for longer than allowed, the module that noticed stores U0100.
That detail matters. U0100 is often stored in the transmission, ABS or body module, not in the engine computer itself. A U0100 that sits in one module with the engine running fine is a very different job from a U0100 with a car that won’t start.
Is it safe to drive?
Short trips only, and only if the engine starts and runs normally. When other modules lose the engine computer’s data, the transmission may fall back to one gear, stability control can switch off, and gauges can freeze or drop to zero. That makes highway driving a bad idea until it’s checked.
The answer becomes no if the car won’t start, stalls, or loses power while driving. A stall at speed takes away power assist on steering and brakes, so don’t risk it; have it towed. If the only sign is a stored code after a battery change or jump start, the car may be fine, which is covered below.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Weak battery or low voltage during cranking | Battery load test; code appeared on a cold start or after a jump | $0 if it only needed a charge; $150–$350 for a battery |
| Loose or corroded battery terminals, engine or body grounds | Voltage drop test across cables and ground straps; green or white crust at the posts | $0–$120 |
| Blown fuse or failed relay feeding the engine computer | Check power and ground at the ECM connector with a meter | $0–$50 |
| Damaged or wet network wiring or connectors | Resistance across the two network pins at the diagnostic port (about 60 ohms on most cars, battery disconnected); inspect connectors for water or green corrosion | $100–$250 to diagnose; $100–$250 to repair |
| Engine computer software fault | Shop checks for a bulletin and reflashes | $100–$200 |
| Failed engine computer | Confirmed power, ground and network at the module, still no communication | $800–$1,800 with programming |
What to check first
- Test the battery and charging system. A healthy battery rests around 12.6 volts and the running voltage usually sits between about 13.5 and 14.7. Our battery keeps dying guide walks through it.
- Clean and tighten the terminals and grounds. Check the engine-to-body ground straps too; a single bad ground can knock modules offline.
- Check the fuses and relays that feed the engine computer. The owner’s manual or fuse box lid names them.
- Read codes from every module, not just the engine. The pattern tells a shop where to look: U0100 in one module points at that module’s wiring; U0100 in several at once points at the engine computer’s power, ground or connector.
- Clear it and drive only if the car runs normally. If it doesn’t come back after a week, it was likely a voltage dip.
- Ask about bulletins before a module is ordered. The manufacturer bulletins lean toward software: several Ford bulletins cover U0100 stored in other modules during self-tests, and a 2023 Toyota Corolla bulletin says a brake-computer U0100 can’t be cleared but doesn’t affect operation. A few do call for hardware, such as the Chevrolet Trax bulletin replacing the body and power-steering modules, and a Volkswagen Passat bulletin covering a crank-no-start with lost engine computer communication.
If the car cranks but won’t start, see car won’t crank but has power for the basic checks.
86 owner complaints filed with NHTSA mention U0100. The Chevrolet Silverado 1500 leads with 10 and the Dodge Charger follows with 7, five of them 2006 models. These are owner reports, not confirmed defects.
What it costs
The cheap outcome is a battery charge, a cleaned terminal or a fuse, $0–$120. The common outcome is a new battery or a wiring or connector repair, $100–$350 plus diagnosis. The expensive outcome is a replacement engine computer, $800–$1,800 with programming, and it should be the last thing replaced, not the first. Paying $100–$250 for network testing is what keeps you from buying a module you didn’t need. If you want help reading your freeze-frame data before you book, ask our AI mechanic.
Related codes
U0100 often shows up with other lost-communication codes: U0101 for the transmission computer, U0121 for the ABS module and U0140 for the body computer. Several at once usually means a shared power, ground or network wiring problem, not several failed modules.
What owners report to NHTSA
86 complaints filed with NHTSA mention U0100, 83 of them since 2015, across 52 different models. The component owners most often filed it under: electrical system.
Most-reported models
| Vehicle | Complaints | Model years reported |
|---|---|---|
| CHEVROLET SILVERADO 1500 | 10 | 2007, 2009, 2019, 2020, 2021 |
| DODGE CHARGER | 7 | 2006, 2014, 2018 |
| CHEVROLET EQUINOX | 4 | 2006, 2010, 2012, 2019 |
| FORD FOCUS | 4 | 2012, 2014, 2017 |
| GMC SIERRA 1500 | 3 | 2016, 2019, 2023 |
| DODGE DURANGO | 3 | 2014, 2015, 2017 |
| CHEVROLET MALIBU | 3 | 2008, 2020, 2023 |
Manufacturer service bulletins that name U0100
165 technical service bulletins and manufacturer communications filed with NHTSA mention U0100, from 19 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- FORD F-150 (2024), bulletin 10254172: Some 2024 F-150 non-Lightning vehicles equipped with minor feature codes C2BAW and FDDAA may exhibit an intermittent Trailer Brake System Fault See Manual message in the instrument panel cluster (IPC), inoperative traile…
- SUBARU BRZ (2022, 2023, 2024), bulletin 11009691: "This bulletin announces availability of new reprograming files for the Engine Control Module (ECM) developed to remedy the following DTCs: P0016: Crankshaft Position - Camshaft Position Correlation Bank 1 Sensor A P0017…
- FORD F-450 SD (2023), bulletin 10233834: Some 2023 F-Super Duty vehicles may set diagnostic trouble code (DTC) U0100:87 in the telematics control unit (TCU) during a diagnostic self-test. This DTC sets when the TCU does not detect a valid signal from the powert…
- FORD F-250 SD (2023), bulletin 10235318: Some 2023 F-Super Duty vehicles may set DTCs B109F:55, B1330:9A, U0252:87, U0198:87, U0184:87, U0155:87, U0256:87, and/or U0100:87 in the BCM when a diagnostic self-test is performed. This may be due to an incorrect sequ…
- LINCOLN CORSAIR (2023), bulletin 10237587: Some 2023 Corsair vehicles may exhibit a continuous memory diagnostic trouble code (CMDTC) U0100 stored in the audio front control module (ACM) and/or instrument panel cluster (IPC) after a self test is performed with Fo…
- TOYOTA COROLLA (2023), bulletin 10241413: RL: Due to certain programming in the brake ECU, under certain conditions, DTC U0100 is stored and cannot be cleared even though a technician attempts to do so. This condition does not affect the vehicle operation.
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can a bad battery cause U0100?
Yes, and it's one of the most common reasons. When battery voltage dips during cranking, modules can drop off the network for a moment and log U0100 against the engine computer. Test the battery and charging system before anything else, especially if the code appeared on a cold morning or after a jump start.
Can I clear U0100 and keep driving?
If the car starts and runs normally, clearing it once is a reasonable test. If it doesn't come back after a week of normal driving, it was most likely a voltage dip. If it returns, or comes with stalling, a no-start or several warning lights, the network or the engine computer needs a proper diagnosis.
Does U0100 mean my ECM is bad?
Not usually. The code says the messages stopped arriving, not why. Power, ground, a fuse, a damaged network wire or a wet connector can all silence a working engine computer. The module is the last thing a good shop replaces, after it has confirmed power, ground and network wiring are good at the module's connector.
What are the symptoms of U0100?
It depends on how much of the car lost contact. Common signs are several warning lights at once, gauges that drop to zero or flicker, a transmission stuck in one gear, and on some cars a crank-no-start or stalling. Sometimes there are no symptoms and the code sits in another module's memory from an old event.