U0100 — Lost Communication With ECM / PCM (MG Context)
Standard meaning: another module stopped hearing from the engine control module on the CAN network. U-codes describe communication, not the modules themselves — and that makes context everything: a U0100 logged during a flat-battery event or jump start is very often a ghost, recorded in the seconds the network browned out. On MGs this matters doubly, because the documented 12V patterns mean plenty of MG owners meet their first U-code right after a 12V drama.
Ghost or real? The triage
| Situation | Reading |
|---|---|
| Appeared alongside a flat/replaced 12V battery, jump start, or P0562 | Likely a ghost. With the battery healthy again, the stored code can be cleared once (photograph everything first). If it stays gone — it was the brown-out. |
| Returns repeatedly with a healthy 12V system | Real network problem: chafed wiring, corroded connector, failing module. Workshop, with the freeze-frames intact. |
| Comes with visible symptoms — no-start, dead gauges, warning-light carnival | Real and current. Don’t clear anything; that history is the diagnosis. |
MG-specific context
Our documented owner-reported patterns on MG’s current range are heavy on electronics: 12V drain and charging faults (ZS EV, MG4, HS), infotainment freezes and delayed cameras. Infotainment glitches alone are not U0100 material — the screen freezing is its own documented annoyance, not evidence of a powertrain-network fault. Where the two meet is the 12V system: a browning-out battery produces both screen weirdness and ghost U-codes, which is why the 12V health test comes first in every one of these stories.
Safe owner checks
- History honestly reconstructed: any flat battery, jump start, charger left connected, or accessory install shortly before the code?
- 12V battery age and terminal condition (engine off; 12V only — never orange HV cabling).
- Aftermarket devices on the OBD port itself (trackers, insurance dongles) — a known source of network mischief on any brand. Remove and re-test.
When it’s a workshop job
Persistent U-codes need network-level diagnosis: powered-and-communicating checks module by module, wiring inspection at connectors that live in harsh spots. It’s methodical work — the parts-cannon fails hardest of all on network faults, because the module that “went silent” is frequently the victim, not the culprit.