U0100 (lost communication with the engine computer): what it means

What does U0100 mean?

U0100 is the generic code for lost communication with the engine or powertrain control module, the computer that runs the engine (ECM) or the engine and transmission together (PCM). General Motors and FCA use the same wording:

• General Motors: "Lost Communication With ECM/PCM" (GM 08-07-30-021H)
• FCA (Jeep): "Lost Communication With ECM/PCM" (FCA 18-006-23)

The "U" means it is a network code. A modern car's modules (engine, transmission, brakes, steering, body, instruments) share data over a wired network. GM calls its version the "High Speed GMLAN Serial Data Bus". Each module expects to hear regular messages from the others. When one stops hearing from the engine computer, it logs U0100.

That is why U0100 is often stored in a module other than the engine computer. A GM steering bulletin for 2014 Silverado and Sierra LD pickups, for example, has technicians read the codes stored in the electric power steering module and lists U0100 there as "Loss of communication with other modules"; it adds that the steering gear doesn't need replacing if that code is set (GM PIT5213).

Why U0100 can turn on so many warning lights

When a module loses contact, it can no longer receive what it needs, so warnings and default modes follow. California's OBD rules also require that on 2010 and newer vehicles, if the instrument panel stops receiving the other computers' requests to light the check-engine light "(e.g., communication is lost)", the light must default to on (CARB § 1968.2, section (d)(2.2.4)).

GM's bulletin on network faults in its 2007–2014 full-size trucks and SUVs with the 6-speed automatic or two-mode hybrid, plus the Corvette, XLR and 2008–2009 H2, lists what drivers may notice (GM 08-07-30-021H):

• the check-engine light and other instrument warning lights on;
• the transmission not shifting, or staying in 2nd gear;
• the engine not cranking, intermittently;
• the engine stalling (on the Corvette and the hybrid full-size trucks);
• gauges fluctuating, door locks cycling on their own, chimes sounding;
• the engine cooling fan running continuously and running the battery down.

A later GM bulletin for 2014–2020 full-size trucks and SUVs adds reduced or lost power steering assist on electric-steering models, the instrument cluster, radio and climate display going blank, "Engine stall" and "No Start" (GM 18-NA-161).

Is it safe to drive with U0100?

Our advice, based on the symptoms above: don't treat U0100 like a routine check-engine code.

• If the car has stalled, won't shift, or the steering has become heavy, don't drive it. Have it towed or looked at where it is.
• If it drives normally and the code is only in history, have it diagnosed soon anyway. An intermittent network fault can return while you are driving, and the bulletins above list stalling among its effects on some models.
• If the engine won't crank at all, start with the battery. See car won't start and just clicks.

What causes U0100?

GM's network bulletin is explicit about the order of work: "DO NOT replace any control module, wiring harness or component until you have first isolated the cause of the condition." A failed engine computer is possible, but the bulletins put wiring, connections and power first. Our inference: an engine computer that has lost its own power or ground also goes silent on the network, which is one more reason to check the supply before the module.

What you can check yourself

1. Unplug anything in the diagnostic port. GM's advice for aftermarket devices is to remove the device and see whether the condition goes away. We'd apply that to any plug-in device: insurance and fleet trackers, and Bluetooth OBD dongles, ours included.
2. Check the battery and its connections. A weak battery or a loose, corroded cable can upset every module at once. Measure the resting voltage (see the battery voltage chart) and look at the terminals. Wear eye protection around the battery.
3. Note every code from every module. GM's 18-NA-161 tells technicians to "Record history and current codes from all modules before clearing". Several U codes at once point to a shared cause.
4. Ask a dealer about bulletins and software updates for your exact vehicle.

A note on scan tools (our inference): a basic reader that asks only for emissions-related codes may not show a U0100 stored in the brake, steering or body module. A shop's full-system scan tool reads every module. If a basic reader shows nothing but warning lights are on, that doesn't mean nothing is wrong. If the scan tool can't connect at all, see why an OBD2 scanner won't connect.

Don't just clear the codes. A network fault that is still there will come back, and clearing also wipes history a technician could use. See pending, stored and permanent codes and check engine light on: can I keep driving?