U0100 on GMC: How to Diagnose & Fix
Lost Communication with ECM/PCM
2 GMC models · 3 documented issues · 3 critical · $80-$900 repair
U0100 on GMC by model: symptoms, diagnosis and fix
Sierra 3500HD owners towing heavy loads report intermittent trailer brake output loss, warning messages, or erratic gain behavior from the integrated trailer brake controller. NHTSA complaints and GM service bulletins/diagnostics commonly point to module/software issues, poor connector pin fit/corrosion at the 7-way, or wiring faults that cause the system to disable trailer brakes unexpectedly.
Common Symptoms
- "Service Trailer Brake System" message
- No trailer brake output or intermittent braking
- Trailer brakes grab or pulse unexpectedly
- Gain setting changes but braking force does not
- Trailer disconnected message with trailer connected
How to Fix
Inspect and clean the 7-way connector, check for spread pins/corrosion, and verify trailer wiring integrity. Scan for trailer brake control module DTCs and apply GM software updates/reprogramming when applicable; replace the trailer brake control module if it fails self-tests. Confirm proper ground paths and repair chassis/connector grounds as needed.
Sources for this issue (2)
On 2020+ Sierra 3500HD (T1 platform) with GM's electronically controlled hydraulic Digital Steering Assist, owners report intermittent loss of steering assist and warning messages such as "Steering Assist Reduced". NHTSA complaints and GM service information commonly point to power steering control module/software issues, wiring/connector concerns, or steering gear faults that can occur without prior symptoms.
Common Symptoms
- "Steering Assist Reduced" or "Service Power Steering" message
- Heavier steering effort, especially at low speeds
- Intermittent assist loss after startup or during turns
- EPS warning light and stored steering control DTCs
- Assist returns after key cycle
How to Fix
Scan for power steering control module DTCs and follow GM SI diagnostics for the steering gear/module and harness. Perform GM-released software updates/reprogramming when applicable; repair wiring/connectors or replace the steering gear if internal faults are confirmed. Verify battery/charging system health because low voltage can trigger steering assist faults.
Sources for this issue (2)
Owners report a cascade of warning messages appearing simultaneously — 'One Pedal Driving Unavailable,' 'Service ESC,' 'Service Trailer Brake System,' and 'Service Active Steering System' — with the vehicle's speed limited to approximately 36 mph. GM has not published a cause, and in the owner report filed with NHTSA the dealer could not find the source of the fault; the same report also lists '4WD Unavailable' and 'Service Driver Assist System' messages. With the active (rear-wheel) steering faulted, the Hummer EV's CrabWalk feature may also become unavailable. The issue can occur without warning, even while stopped, and a power-down and restart has cleared it in owner reports.
Common Symptoms
- Multiple warning messages appear on the dashboard simultaneously
- Vehicle speed limited to approximately 36 mph
- One Pedal Driving Unavailable message
- Service ESC warning light
- CrabWalk and four-wheel steering features disabled
How to Fix
Pull over safely and perform a complete vehicle power-down: turn off, open and close the door, wait 3-5 minutes, then restart. If warnings persist, the vehicle needs a dealer visit. Do not ignore the speed limitation — driving at highway speeds may not be safe if the ESC system is faulted. Ask the dealer to scan all modules for stored codes and to confirm that every available GM software update has been installed (for the 2024 model, GM Update 653.20 includes fixes to help maintain ESC and One-Pedal Driving availability). Note the date, time and exact messages, because dealers often cannot reproduce an intermittent fault.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
U0100 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does U0100 mean on a GMC?
U0100 stands for "Lost Communication with ECM/PCM." U0100 is a network communication code meaning a module has lost communication with the engine control module / powertrain control module (ECM/PCM) over the vehicle's data bus. The ECM/PCM is the brain that runs the engine, so when other modules can no longer 'talk' to it, the effects can be serious — a no-start, stalling, multiple warning lights, and many other modules logging faults at once. This is almost always a wiring, connector, power/ground, or bus fault, or a failure of the ECM/PCM itself, rather than a single sensor. It should be diagnosed promptly because the vehicle may be undrivable. On GMC it is documented across 2 models: Sierra 3500HD, Hummer EV.
What are the most common causes of U0100?
The most common causes of U0100 are: Loss of power or ground to the ECM/PCM, CAN bus wiring damage, short, or open circuit, Corroded, loose, or damaged connectors at the module, Blown fuse feeding the ECM/PCM, Failed ECM/PCM, Another faulty module pulling down the data bus, Water intrusion or corrosion in a connector/harness. The specific cause varies by vehicle.
How much does it cost to fix U0100 on a GMC?
Published repair estimates on GMC range from $80 to $900 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.