What U0100 means
The other computers in the car have stopped hearing from the engine computer (ECM or PCM). Because that computer runs the engine, the car may stall, not start, or light up several warning lights at once. Most often the engine computer has lost power or ground, or a network wire or connector to it has failed.
How the engine computer detects it
The ECM/PCM "A" has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. As this is the engine controller itself, the engine usually will not start or run while the message is missing. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
- ECM unpowered (fuse, ground, supply wire) Most likely
- CAN wiring to ECM broken Possible
- ECM internally failed Less likely
How to diagnose U0100
- Have the battery and charging system tested, and make sure the battery terminals are clean and tight. Low voltage is a common trigger for network codes.
- Check the fuses and relays that feed the engine computer in the underhood fuse box.
- Inspect the engine and body ground straps and the engine computer's ground wires for corrosion or loose bolts.
- With the ignition off, check the engine computer's connectors for corrosion, water or pushed-out pins.
- With a scanner that reads all modules, a shop can see whether the engine computer answers at all and then test its power, ground and network wires at the module.
- Replace the engine computer only after power, ground and wiring check out. A new one usually has to be programmed to the car.
Vehicles that report U0100 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Chevrolet Silverado 1500 | 2007–2021 | 10 | 3k | |
| Dodge Charger | 2006–2018 | 7 | 86k | |
| Ford Focus | 2012–2017 | 5 | 40k | |
| Chevrolet Equinox | 2006–2019 | 4 | 29k | |
| GMC Sierra 1500 | 2016–2023 | 3 | — | |
| Dodge Durango | 2014–2017 | 3 | — | |
| Chevrolet Malibu | 2008–2023 | 3 | — |
Counts of owner complaints to the NHTSA that mention U0100 (88 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- U
- Network
- 0
- Generic (SAE) code
- 100
- Specific fault
Questions drivers ask
Can I drive with code U0100?
If the engine stalls, won't start, or several warning lights come on together, don't drive: have it towed. If the code is stored but the car runs normally, it may have been a brief dropout; drive carefully straight to a shop.
Often reported together
Codes that owners mention in the same NHTSA complaints as U0100. They can point to a shared cause, so it's worth checking them together.
- U0101Lost Communication with TCM17 complaints
- P0700Transmission Control System (MIL Request)15 complaints
- U0140Lost Communication with Body Control Module9 complaints
- U0073Control Module Communication Bus "A" Off6 complaints
- U0121Lost Communication with ABS Control Module6 complaints
- U0146Lost Communication with Gateway "A"6 complaints
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.