Why 'lost communication' rarely means a dead module

U0100 reads as lost communication with the engine control module, and on paper that sounds like a computer has failed outright. In practice the module itself is very often intact, and the fault sits somewhere between it and the rest of the network — a wire, a ground, a connector, or the shared data bus losing its footing rather than any single computer dying.

How the network actually fails

Modern cars link their control modules over a shared network rather than wiring each one directly to every other. When one module's signal quality degrades — a chafed wire, a poor ground, a voltage dip — the network's own error-handling can push that module into a protective "bus-off" state, where it stops transmitting rather than risk corrupting the shared line for everyone else. Every other module then reports that it has lost communication with the one that went quiet, even though the quiet module may still be functioning normally underneath.

That is why these codes tend to arrive in clusters rather than one at a time. A single wiring fault upstream of several modules, or a bus-off event on a shared segment, can set U0100, a related U-code for a different module, and a third for the gateway that ties the network together, all from one root cause. Replacing whichever module the first code names does nothing for a fault that was never in that module to begin with.

What clusters with it

These are the codes most likely to turn up in the same scan as U0073, the lost-communication code for the network's high-speed bus rather than one specific module:

That is exactly the pattern a shared bus problem should produce. U0100, U0140 and U0121 each name a different module, but they are showing up together often enough that the common thread is the bus itself, not any one of the named modules.

What the manufacturers' bulletins say

GM has published roughly 30 bulletin communications describing bus-off conditions that produce this exact cluster — U0073, U0100 and U0140 together — with symptoms reported alongside them including loss of power-steering assist, stalling, and instrument or infotainment displays going blank. The bulletins tie the cluster to the network condition rather than to any of the named modules individually.

Bulletins are revised or superseded as manufacturers learn more; verify the current version for your vehicle through NHTSA's public database or your dealer.

Where to look before replacing a module

Start with the wiring harness at points where it flexes or runs near heat — a chafed insulator shorting intermittently produces exactly this symptom and is invisible until someone looks for it. Check chassis and module grounds next; a poor ground reads to the network as noise, not as a clean disconnect. Have the charging system and battery voltage checked too, since a voltage dip under load can push a marginal module into the same protective bus-off state. Only after wiring, grounds and voltage come back clean does a gateway or individual module diagnosis make sense.

Can you keep driving?

It depends entirely on what came with the codes. If nothing changed about how your car drives — no steering, braking or stalling symptoms, just the light and the code — it is reasonable to get it scheduled without treating it as an emergency. If steering assist, stalling or blanked displays showed up with it, treat that as the car telling you the network fault has reached something safety-relevant, and stop rather than diagnose from behind the wheel.