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Home / DTC / U028B — Lost Communication With Active Grille Air Shutter Module C

U028B — Lost Communication With Active Grille Air Shutter Module C

Detailed page for trouble code U028B.

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Code

U028B

Generic U — Network/User

Lost Communication With Active Grille Air Shutter Module C

Brand: Generic
Type: U — Network/User
Views: UK: 23 EN: 27 RU: 21
AI status
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Page language: EN

Causes

  • Open or shorted CAN bus wiring (CAN High / CAN Low) to the module
  • Module power (fuse, supply) or ground missing at the shutter module
  • Corroded or loose connector at the module
  • Failed Active Grille Air Shutter Module C
  • Defective termination resistor or bus wiring causing reflections/noise
  • Other module on same CAN bus causing bus errors

Symptoms

  • DTC U028B stored and related MIL or message lamp may be illuminated depending on vehicle
  • Active grille shutter C will not respond or remain in default position (stuck open/closed)
  • Reduced aerodynamic control or cooling control related warnings/messages
  • Other modules may show additional U-codes or reduced functionality
  • Intermittent or permanent loss of grille shutter operation

What to check

  • Connect a professional scan tool and confirm U028B and any additional communication-related codes; note freeze frame / timestamp
  • Check battery voltage and condition; ensure correct system voltage during tests
  • Inspect module connector for corrosion, bent pins, pushed out seals or missing retention
  • Check fuses and power/ground circuits for the shutter module (pin-power-ground present with key on)
  • Visually inspect CAN wiring harness along routing for damage, pinches, or repairs
  • Measure CAN bus termination resistance between CANH and CANL (should be approx. 60 ohms with ignition off, vehicle specific)

Signal parameters

  • CAN bus nominal idle voltages: CAN High ≈ 2.5 V, CAN Low ≈ 2.5 V (vehicle dependent)
  • Dominant state typical differential ≈ 1.5–2.5 V between CANH and CANL
  • Expected bus data rates commonly 125 kbps, 250 kbps, or 500 kbps (confirm vehicle-specific)
  • Typical termination resistance across CANH–CANL ≈ 60 ohms (two 120Ω in parallel) when harness connected and ignition off
  • Module should send periodic status messages; absence = timeout/lost comm

Diagnostic algorithm

  1. Verify code is current: read codes, note freeze frame, and check for related U-codes.
  2. Attempt to communicate with Active Grille Air Shutter Module C via scan tool. Confirm no response or intermittent response.
  3. With key ON (engine off), verify module supply voltage and ground at the module connector using a DMM. Repair any missing power/ground.
  4. Inspect and reseat the module connector; check pins for corrosion, bent terminals, or water intrusion.
  5. Check fuses and fusible links that supply the grille shutter module; replace if blown and re-test.
  6. Measure resistance between CANH and CANL at the module connector and at another node; verify proper termination (~60 Ω).
  7. Using an oscilloscope or CAN bus analyzer, observe CANH/CANL waveforms for noise, bus loading, or missing traffic. Look for stuck dominant or missing recessive levels and verify message IDs.
  8. Wiggle test wiring harness while monitoring communication to try to reproduce intermittent faults; isolate any damaged sections and repair wiring/connectors.
  9. If wiring and power are good and the bus is healthy, try replacing or reprogramming the Active Grille Air Shutter Module C per manufacturer procedures.
  10. Clear codes, perform functional test of grille shutters, and road test to confirm repair. Re-scan for return of codes.

Likely causes

  • Loose/corroded connector at the active grille shutter module
  • Damaged CANH/CANL wires (pinched, chafed, short to chassis or battery)
  • Blown fuse or missing battery feed / ground to the shutter module
  • Failed shutter module electronics
  • Missing or incorrect CAN termination (open/shorted resistor)

Fault status

⚠️ Status
Lost Communication With Active Grille Air Shutter Module C — CAN message timeout or no response from module
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

Similar codes

U0100 U0121 U0140 U0155 U028A
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