U0100 Lost Communication with ECM/PCM on Smart Fortwo 451 / 453
U0100 is lost communication between modules and the engine ECU over the CAN bus. On Smart it almost always follows a flat battery or a jump-start event. Check battery health and CAN bus connectors before you suspect the ECU itself.
Typical Symptoms
- Check engine light with code U0100 (often with several other U-codes)
- Multiple warning lights all on at once (ABS, ESP, transmission, airbag)
- No-start or starts and immediately dies
- Cluster not communicating — gauges dead or stuck
- Symptoms appeared after a flat battery, jump-start, or battery replacement
- Codes in multiple modules at the same time
What it means
U0100 is a network code: lost communication with the engine control module over the CAN bus. The CAN bus is the common wiring backbone that lets all the control modules on the car talk to each other — ECU, TCM, ABS, airbag, cluster, BCM all share it. When one module can't reach another, you get a U-code from whichever module noticed.
On a Smart, U-codes have a specific signature: they almost always follow a flat battery or a jump-start event. The combination of low voltage during the event and the voltage spike that can come with a careless jump confuses modules and sometimes corrupts what they were doing at the time. The good news is the fix is usually battery-related rather than module-replacement. The bad news is that several U-codes setting at once — U0100 with U0101, U0121, U0140 and others — looks scary on a scan tool even when the cause is simple.
This page is tagged shop because once cheap-first checks are exhausted, real CAN diagnosis needs a scope or a Smart-capable scan tool. But the cheap-first checks are absolutely worth doing yourself first.
Likely causes, cheapest first
- Weak or fully discharged battery. The dominant cause on Smart. Voltage dips below what the modules need to stay synchronized and you get a stack of U-codes after the event.
- Battery terminal corrosion or loose clamps. Free to check. Resistance at the terminal looks like low voltage to the modules downstream.
- Aftermath of a jump-start. A jump that hooked up backwards even briefly, or a jump from a much larger vehicle's running engine, can spike voltage hard enough to upset modules. Usually clears on its own once the battery is back to healthy.
- CAN bus connector loose at the ECU or another module. Less common, but worth a look if you've recently had something apart in the engine bay or under the dash.
- ECU itself failed. Rare and the worst case. Confirmed only after everything else is ruled out, and replacement requires coding to the car.
DIY check steps
- Read all stored codes in every module. A scan tool that only reads the engine ECU isn't enough for a U-code — you want to see what every module on the car is reporting. The pattern across modules tells you whether one module is silent or several are talking past each other.
- Test the battery. Resting voltage, cranking voltage, and a load test. If it's marginal, replace it. On a Smart that alone clears a lot of U-code clusters.
- Clean the battery terminals. Pull both cables, wire-brush posts and clamps until shiny, reattach tight.
- Battery disconnect for thirty minutes, then reconnect. This forces every module to reinitialize on its next wake. Several U-codes that came on after a flat battery will clear after this reset alone.
- Inspect the ECU connector if you've had any work done in the engine bay recently. A pin that wasn't fully seated when something was reassembled can be the cause.
When to call a shop
If the battery is healthy, the terminals are clean, and U0100 keeps coming back, that's CAN bus diagnosis territory. A Smart-experienced shop with the right scan tool — MB Star, Xentry, or a community AutoFlash — can talk to each module individually and identify whether the engine ECU has actually gone silent or whether the bus itself has a wiring problem. ECU replacement is the worst case and it's a coded part — it has to be programmed to the car after install, which is a dealer or specialist job. Don't go there until the cheap stuff is genuinely ruled out.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Battery (load-test before replacing) | $100-180 | Search Google |
| Battery terminal cleaner / wire brush | $5-15 | Search Google |
| ECU / control module connector cleaner | $10-20 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Service manual on Manualslib — external mirror (we don't host this specific document).
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
- Evilution: OBD-II error code reference
- OBD-codes.com: U0100 reference
- SmartCarOfAmerica: U0100 search
Stuck on this one?
SmartDiag-AI runs through the cheap-first checks with you, weighted to community-known patterns for your exact model. The link below pre-fills the code and model.