OBD codes / U0100
U0100
SeriousLost Communication With ECM/PCM A
Treat U0100 as urgent. Losing communication with the engine computer can cause a no-start or sudden stalling, and multiple systems may misbehave at once. It's not a code to drive around with — have it diagnosed before relying on the car, and don't be surprised if it won't start.
What this code means
Modern cars use a data network (CAN bus) so the computers can talk to each other. U0100 means a module lost communication with the engine/powertrain computer (ECM/PCM). This is a network or electrical fault, not a specific part failure: common causes are damaged bus wiring or a corroded connector, a blown fuse, a power/ground problem at the ECM, or the ECM itself failing. Because it depends entirely on where the communication breaks down, the repair can be anything from a cheap connector fix to a module replacement, so it must be diagnosed.
Symptoms you might notice
- Multiple warning lights at once
- No-start or stalling
- Gauges or accessories behaving erratically
- Other 'U' communication codes stored alongside it
What it costs to fix
Varies by vehicle — get it diagnosed.
The repair cost for this code depends too much on your exact vehicle and the underlying cause to quote a meaningful range. Confirm the cause first so you only pay for the fix you actually need.
Diagnose U0100 for my vehicle →How to fix U0100
Advanced — usually a shop job unless you’re well equipped
- Test the battery and clean both terminals
Low system voltage is a notorious cause of modules dropping off the network, and it's the cheapest thing on this list. Load-test the battery, clean the terminals, and check the main ground straps from battery to body and body to engine for corrosion or looseness.
- Check every fuse that feeds the ECM
Pull and inspect the ECM/PCM fuses in both the underhood and interior fuse boxes — there are often several. A single blown fuse can silence the engine computer entirely, and the fix costs pocket change.
- Inspect the ECM's connectors and surroundings
Look at the ECM's connectors for green corrosion, water intrusion, or backed-out pins, and check for evidence of rodent-chewed wiring nearby. Water finding its way into an ECM connector is a classic cause of lost communication.
- Scan the whole vehicle to map who's missing
Run a full-system scan and note which modules respond and which store U0100. Every module complaining about the ECM points at the ECM's power, ground, or bus connection; only one module complaining points suspicion back at that reporting module instead.
- Have the CAN bus itself tested before replacing anything
A shop will check bus resistance at the diagnostic connector (a healthy twisted-pair CAN network reads about 60 ohms) and probe the wiring for shorts or opens. Only after the network and the ECM's supplies test good should ECM replacement — which usually requires programming to the vehicle — be on the table.
DIY or shop? The battery, grounds, fuses, and a visual on the ECM connectors are meaningful DIY steps that resolve a real share of these codes cheaply. Past that, this is shop work by nature: CAN-bus diagnosis requires a wiring diagram, a meter or oscilloscope on the right pins, and the judgment to avoid the expensive trap of replacing a module that a five-dollar wiring repair would have fixed. A replacement ECM also typically needs dealer-level programming, which a driveway can't supply.
Tools for the DIY steps: Multimeter · Battery load tester or voltmeter · Fuse puller and spare fuses · Full-system OBD-II scan tool · Wire brush for ground connections
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for U0100.
Frequently asked
Can I drive with U0100?
Not reliably. Loss of communication with the engine computer can cause stalling or a no-start and can affect several systems at once. Have it diagnosed promptly rather than driving on it.
What does U0100 mean in plain terms?
The car's computers talk over a data network, and U0100 means one of them stopped hearing from the engine/powertrain computer. It's a wiring, power, or module communication problem rather than a single mechanical part.
How much does it cost to fix U0100?
It varies widely — a wiring or connector repair is inexpensive, but a failed ECM/PCM (often needing programming) is costly. Because the cause ranges so much, it has to be diagnosed before anyone can quote it.
Is U0100 the ECM itself failing?
Sometimes, but not always. Damaged bus wiring, a corroded connector, a blown fuse, or a power/ground fault are common and far cheaper than a computer. A proper diagnosis checks those before condemning the module.
Seeing U0100 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose U0100 for my vehicleSymptoms that trigger U0100
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like U0100 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.