U1900: How to Diagnose & Fix — CAN Communication Bus Fault
2 vehicles · 2 makes
How to diagnose and fix U1900: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Saab 9-3 · 2003–2011
Second-gen 9-3 (2003-2011) sunroof drains clog with debris (pollen, leaves). Water then overflows into the headliner, runs down the A-pillars, soaks the carpet, and pools in the front footwell — directly under the DICE (Driver Information Central Electronics) and TWICE modules. Wet modules can short and cause a cascade of weird electrical faults (no-start, dead instrument cluster, ABS faults, etc.).
Common Symptoms
- wet carpet
- no-start intermittent
- dead dash
- multiple electrical warnings
- mildew smell
How to Fix
Clear sunroof drains annually (compressed air from above, fishline gently from below). If carpet is wet, dry IMMEDIATELY — remove driver carpet and inspect the modules. Once a module is corroded the only fix is replacement and recoding ($300-$1,200). Aftermarket dehumidifier packs in footwell help in humid climates.
Ford Mustang Mach-E · 2021–2025
The Mach-E SYNC 4A infotainment system with the 15.5-inch touchscreen experiences freezing, black screens, and spontaneous reboots while driving. Since the touchscreen controls nearly all vehicle functions (climate, charging, drive modes), a frozen or rebooting screen can leave the driver unable to adjust settings or use navigation. The system may take 30-90 seconds to reboot, during which the backup camera and climate controls are unavailable. Ford has released multiple OTA updates to address stability.
Common Symptoms
- 15.5-inch screen goes black while driving
- Screen freezes and does not respond to touch
- System reboots spontaneously showing Ford logo
- Climate controls unavailable during reboot
- Backup camera blank when screen is rebooting
- Apple CarPlay/Android Auto disconnects
How to Fix
Install all available OTA software updates — Ford has released numerous stability fixes. Perform a soft reset by pressing and holding the power button on the center screen for 10 seconds. For a full system reset, disconnect the 12V battery for 2 minutes. If freezing persists after updates, the APIM (Accessory Protocol Interface Module) may need replacement under warranty. Avoid running many apps simultaneously to reduce memory pressure.
Sources for this issue (2)
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U1900 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 U1900 mean?
U1900 stands for "CAN Communication Bus Fault." U1900 is a manufacturer-specific network code (commonly Ford) indicating a CAN communication bus fault, often a receive error. The CAN bus is the high-speed network that lets control modules such as the powertrain computer, ABS module, and instrument cluster share data. This code is set when a module stops receiving expected messages from one or more modules on the bus. Because so many systems depend on the network, a bus fault can trigger multiple warning lights and erratic behavior across systems.
What are the most common causes of U1900?
The most common causes of U1900 are: Damaged, corroded, or loose CAN wiring/connectors, A failed or offline module pulling down the bus, Short to power or ground on a CAN line, Open circuit or broken CAN harness, Failed instrument cluster or ABS/PCM module, Low system voltage or weak battery, Water intrusion into a connector or module. The specific cause varies by vehicle.
How much does it cost to fix U1900?
Repair costs vary widely depending on the root cause and vehicle.
Which vehicles are affected by U1900?
Au7o has documented U1900 across 2 vehicle models from 2 manufacturers: Ford, Saab.
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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.