U1304

Code

HUMMER U — Network/User

Loss Of UART Communications

Brand: HUMMER

Type: U — Network/User

Views: UK: 9 EN: 12 RU: 8

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Page language: EN

Related module functions are inoperative or intermittent (e.g., instrumentation, telematics, ABS/traction functions depending on affected module)
Multiple communication-related DTCs present or cascading network errors
Illumination of MIL/warning lights tied to the affected modules
Features that rely on that serial link fail to respond
Intermittent faults that clear and return after driving or vibration

Use a scan tool capable of reading manufacturer U-codes to confirm U1304 and list related codes
Visually inspect connectors and harnesses for corrosion, bent pins, water damage, chafing, or repair splices
Verify battery voltage (>=12.4V) and stable power during diagnosis
Check module fused power feed and ground(s) for proper continuity and resistance to chassis
Backprobe module UART TX/RX pins with an oscilloscope or logic analyzer to verify activity
Check for wiring continuity and shorts between TX/RX and power or ground

Common UART frame: 1 start bit, 8 data bits, no parity, 1 stop bit (8N1) — actual format is manufacturer dependent
Typical baud rates can range (example) 9,600; 19,200; 38,400; 57,600; 115,200 bps — verify vehicle spec
Logic voltage levels typically 3.3V or 5V TTL/CMOS (verify module pinout)
Idle line state is normally HIGH; TX toggles during data transmission
Signals are point-to-point (TX from one module to RX of another); no CAN-style bus arbitration
Suspect pins: TX (transmit), RX (receive), VCC (module power), GND (module ground)

Retrieve all stored and pending DTCs and note when U1304 first occurred. Record related U- and P-codes and freeze frame data.
Inspect wiring and connectors for the modules listed in freeze frame / network map. Look for corrosion, bent pins, water intrusion, or service repairs.
Verify battery condition and measure supply voltage at the affected module’s power pin and continuity to fuse. Check dedicated grounds for low resistance to chassis.
Identify the TX/RX pin locations from the vehicle service data. Backprobe with an oscilloscope or logic analyzer while operating the system or cycling ignition to confirm signal presence and valid UART frames.
If no activity, check continuity of TX/RX wiring back to connector/junction and for shorts to power/ground. Wiggle harness while observing scope to find intermittent breaks.
If signal is present but corrupt, compare waveform timings and voltage levels to factory spec. Look for noise, dropped bytes, wrong baud, or framing errors.
Check for and repair connector or wiring faults. If wiring and power/grounds are good, attempt software/firmware reflash or perform module reset per service procedure.
If problem persists and waveform shows no TX from a module, consider swapping with a known-good module (if applicable) or replace the suspected module as per manufacturer guidance.
After repairs, clear codes, confirm restoration of normal UART activity with the scope, and perform a test drive/functional test to ensure no recurrence.

⚠️ Status

Loss of UART communications detected between control module(s) — serial link inactive, intermittent, or corrupted.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours

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