U0100 Lost Communication With the Engine Computer
Caution
Another module in the car stopped hearing from the engine computer. It can be a one-time glitch or a real network fault. Check the battery, fuses and grounds first.
- Safe to drive?
- Only if the car behaves normally
- Typical cost at a shop
- $456-$1,192 RepairPal national averages, parts and labor
- Emissions test
- Network codes do not always turn on the check engine light.
- Carmaker bulletins
- 2 notes with links
What it means
Modern cars run on a network, the CAN bus, where the control modules talk all the time. U0100 is stored by another module, such as the transmission, ABS or body computer, when it stops hearing from the engine computer. The cause can be wiring, power to the engine computer, or the computer itself.
The technical version
Each module expects regular messages from the others. When a module misses the engine computer's messages for longer than its timeout, it stores U0100. The pattern of which modules store it, and which modules still answer a scanner, points to the fault.
Official name: Lost Communication With ECM/PCM "A".
Symptoms
- Sometimes none: an old stored glitch.
- Several warning lights at once.
- Stalling, no start or dead gauges if it is active.
Causes, most likely first
| Cause | How likely | How to tell |
|---|---|---|
| Low battery voltage or poor charging | most common | Low module voltage, slow cranking. |
| A blown fuse or bad ground for the engine computer | common | The engine module does not answer a scanner. |
| Damaged network wiring: chafing, rodents, corrosion | common | The code is in several modules at once. |
| Aftermarket electronics on the network | less common | Appeared after an install. |
| A failing engine computer | rare | Everything else tests good. |
How likely is our ranking from repair data and common practice, not a published rate.
What the live data shows
The pattern of who answers is the diagnosis. Polling every module shows which ones respond. U0100 in several modules at once points to the network or the engine computer's power. Module voltage matters too: the whole network misbehaves when voltage sags. If the engine module does not answer the scanner at all, that is the finding.
What to check first, in order
-
Test the battery and clean and tighten its terminals and main grounds.
The cheap, common cause.
What the data shows: Module voltage around the normal charging range with the engine running.
Cost: $456-$493 at a shop (Battery replacement, RepairPal)
-
Check the engine computer's fuses.
No power, no messages.
What the data shows: Does the engine module answer?
Cost: free
-
Scan all modules and note who stored U0100.
Several modules point to the network.
What the data shows: Module list and codes.
Cost: no cost if you have a scanner
-
Inspect wiring and unplug recent aftermarket electronics.
Chafe and add-ons are common.
What the data shows: No reading needed.
Cost: free to look
-
Professional network diagnosis before any computer replacement.
Replacing the computer is the last step.
What the data shows: Not a scanner step.
Cost: $1,122-$1,192 at a shop (Engine control module (ECM) replacement, RepairPal)
Is it safe to drive?
Caution
Only if the car behaves normally. An active network fault can shut down systems while driving.
Unpredictable is the honest word. Some cars store U0100 from a short glitch and drive normally; others stall, will not start or lose major systems while driving. If the code is current and the car has symptoms, do not trust it on a highway until it is diagnosed. If it is an old stored code and everything works, have it checked without panic.
Stop driving and get help if
- The engine stalls or cuts out.
- Many warning lights come on together.
- Gauges go dead.
What it costs
| Repair | Typical cost |
|---|---|
| Battery replacementRepairPal, updated September 3, 2026 | $456-$493 |
| Engine control module (ECM) replacementRepairPal, updated September 3, 2026 | $1,122-$1,192 |
National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner.
Can I clear it? Will it come back?
Clear it and see if it returns. A code that never returns was likely a glitch, for example from a weak battery.
Will it fail an emissions test?
Network codes do not always turn on the check engine light. If the light is on, most OBD emissions tests fail the car.
Carmaker notes
Service bulletins and warranty extensions that name this code. Ask a dealer to check your VIN.
-
Chevrolet 2015-2019 Trax 1.4L
GM bulletin 19-NA-113 (May 2019): U0073, U0100, U0101 or U0121 from the engine harness chafing against the bulkhead.
GM 19-NA-113 via NHTSA static.nhtsa.gov
-
GM 2007-2012 full-size trucks and SUVs with 6L80 or 6L90
GM bulletin 08-07-30-021G (March 2012) lists U0100 with P0700 and loss of network communication.
GM 08-07-30-021G via NHTSA static.nhtsa.gov
Related codes
Also related, no guide yet: U0101, U0121.
How Obie handles U0100
Obie asks every module that answers, reads module voltage and shows which modules stored U0100, so you can tell a glitch from a network fault. Obie is part of ScanMyCar Pro. Codes and live sensors are free.
Questions people ask
Can a bad battery cause U0100?
Yes. Low voltage is one of the most common causes. Test the battery and clean the terminals first.
Is U0100 serious?
It can be. If the car stalls or shows many warnings, do not drive it far. An old stored code with no symptoms is less urgent.
Do I need a new engine computer?
Rarely. Rule out power, grounds and wiring first.
Sources
- RepairPal: U0100 code page repairpal.com
- RepairPal: Battery replacement (updated September 3, 2026) repairpal.com
- RepairPal: Engine control module (ECM) replacement (updated September 3, 2026) repairpal.com
- GM 19-NA-113 via NHTSA static.nhtsa.gov
- GM 08-07-30-021G via NHTSA static.nhtsa.gov
- Texas DPS emissions testing FAQ dps.texas.gov
- Arizona myazcar.com: OBD and the check engine light myazcar.com
A guide, not a diagnosis. This page describes what U0100 generally means across OBD2 cars. What is actually wrong with your car depends on its live data and its history, and any repair decision deserves a proper diagnosis, whether that is you with a scanner or a professional with a lift.
