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Home / DTC / U1940 — Front Display SDC Communication Error | AmFm Antenna circuit Low ( shortcut to GND )

U1940 — Front Display SDC Communication Error | AmFm Antenna circuit Low ( shortcut to GND )

Detailed page for trouble code U1940.

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Code

U1940

HYUNDAI U — Network/User

Front Display SDC Communication Error | AmFm Antenna circuit Low ( shortcut to GND )

Brand: HYUNDAI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Short to ground on AM/FM antenna center conductor or amplifier power feed
  • Damaged or corroded antenna coax connector or wiring
  • Failed AM/FM antenna amplifier (internal short)
  • Faulty front display SDC or head unit communication (CAN bus)
  • Blown/loose fuse or poor ground connection
  • Water intrusion in antenna/connector or body harness damage

Symptoms

  • Radio AM/FM reception weak or absent
  • Front display may show radio/antenna error or fail to respond
  • DTC U1940 stored; may be other U-codes present
  • Intermittent audio/static or antenna-related warnings
  • Possible loss of other features controlled by front display if CAN communication impaired
  • Battery drain in rare cases if antenna amplifier power is shorted to ground

What to check

  • Connect a scan tool; record U1940 and any additional U-codes and freeze frame data
  • Visually inspect antenna mast, base, coax, and connectors for damage or corrosion
  • Inspect wiring harnesses and connectors at the antenna amplifier, head unit, and front display SDC
  • Check relevant fuses (radio/antenna) and radio/antenna ground connections
  • Measure antenna amplifier power feed voltage with ignition ON
  • Perform resistance/continuity check from antenna signal center conductor to chassis ground (looking for short)

Signal parameters

  • Antenna power feed (when radio/ignition ON): ~11–14 V (vehicle battery voltage) if powered by 12 V feed
  • Antenna signal line: normally high impedance; DC continuity to ground should be open or >10 kΩ (no short)
  • Antenna coax shield to chassis ground: continuity expected (shield grounded) but center conductor must not be shorted to ground
  • CAN bus idle voltages: CAN_H ≈ 2.5 V (idle) and CAN_L ≈ 2.5 V; during dominant bits CAN_H ≈ 3.5 V and CAN_L ≈ 1.5 V
  • CAN differential idle ≈ 0 V; differential during active ≈ 2.0 V (use a scope or two-channel meter for best view)

Diagnostic algorithm

  1. Read and record all stored codes and live data with a scan tool; note whether U1940 is current or intermittent.
  2. Check for software updates/TSBs related to front display or radio before invasive repairs.
  3. Inspect exterior antenna, mast, and base for damage or water; disconnect antenna connector and inspect pins for corrosion or damage.
  4. With ignition ON, measure voltage on the antenna power feed at the antenna amplifier or head unit harness: confirm expected battery voltage when radio is on. If no voltage, check fuses and radio power circuits.
  5. With antenna disconnected, measure resistance between antenna center conductor pin and chassis ground. If low resistance (close to 0 Ω), locate and repair short in coax or wiring harness.
  6. If short exists, trace harness back from antenna to amplifier/head unit; look for chafing, pinched wiring, or water intrusion. Repair/replace damaged wiring or coax.
  7. If antenna feed voltage present and no short to ground, test the antenna amplifier by disconnecting it and verifying the head unit behavior; replace amplifier if internal short suspected.
  8. Inspect and test CAN bus at the front display SDC and head unit: check CAN_H and CAN_L voltages, continuity, and for bus faults. Repair wiring/connectors if CAN communication is lost or out of spec.
  9. If wiring and antenna/amplifier check good but U1940 persists, swap with a known-good front display/head unit (if available) or test the module on a known-good CAN bus to confirm module fault before replacement.
  10. After repair, clear codes and road-test; verify that U1940 does not return and that radio/display function is restored.

Likely causes

  • Center conductor of antenna coax shorted to chassis ground (broken coax shield exposing center)
  • Antenna amplifier power wire shorted to ground or internal amplifier fault
  • Corroded or pushed-out connector at antenna base or head unit causing low-voltage/short condition
  • Faulty front display/head unit or its CAN transceiver failing to communicate
  • Blown radio/antenna fuse or poor ground at the radio/antenna ground point

Fault status

⚠️ Status
Front display SDC communication error with AM/FM antenna circuit low — antenna feed shorted to ground suspected. Inspect antenna, amplifier, wiring/connectors, fuses and CAN communications.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

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371

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