U028F

Code

Generic U — Network/User

Lost Communication with Active Aerodynamic Feature Control Module C

Brand: Generic

Type: U — Network/User

Views: UK: 21 EN: 28 RU: 20

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Page language: EN

Causes

Open or short in CAN wiring between module C and the network
Poor or missing module power or ground (fuse, relay, connector)
Corroded, loose or damaged connector at the module
Failed or intermittent module (internal electronics)
Network termination resistor open or incorrect value
Other module holding the bus in a dominant state (short to ground/voltage)

Symptoms

DTC U028F stored and possibly MIL or warning lamp illuminated
Loss or non-operation of the vehicle’s active aerodynamic components controlled by Module C (grille shutters, rear spoiler, etc.)
Other network-related modules reporting communication faults
Intermittent or permanent loss of related features
Possible multiple U-codes referencing the same network

What to check

Read and record all active and pending codes from all modules with a capable scan tool
Check for Technical Service Bulletins (TSBs) or wiring diagrams for the specific vehicle
Visually inspect module C connector and harness for damage, corrosion or moisture
Verify vehicle battery voltage (should be within normal operating range) and inspect related fuses/relays
Check power and ground at the module connector pins with a multimeter
Inspect CAN_H and CAN_L wiring for chafing, shorts to ground/voltage, and continuity to the rest of the network

Signal parameters

Typical CAN recessive idle: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both equal)
Typical CAN dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Expected bus termination: ≈ 60 ohms between CAN_H and CAN_L (two 120 ohm resistors in parallel)
Common bus bit rates: 125 kbps, 250 kbps or 500 kbps depending on vehicle (verify service data)
Module message frequency: varies by function — many control modules send periodic status messages from 1 Hz to several tens of Hz; absence of expected periodic frames indicates loss of communication

Diagnostic algorithm

Connect a capable scan tool and read all modules. Note any additional U-codes and freeze-frame data.
Check battery voltage and inspect/replace any blown fuses or faulty relays that supply module C.
Visually inspect the connector and wiring for module C. Repair any obvious damage before further electrical testing.
Backprobe module C power and ground pins. Verify proper supply voltage with ignition on and a good ground connection.
With ignition on, use a scan tool or an oscilloscope to observe CAN_H and CAN_L at the module connector. Confirm recessive and dominant voltages and look for activity (periodic frames).
If the bus is silent at the module but active elsewhere, check continuity of CAN_H and CAN_L from the module connector to the nearest junction/other module.
Measure resistance across CAN_H and CAN_L with power off. Verify ≈60 ohms. If open, trace wiring to find open or removed termination.
Wiggle test wiring harness while monitoring bus activity to detect intermittent shorts or opens.
If wiring and power/grounds are good but module does not communicate, consult service information for module wake-up procedure or programming requirements. Attempt to reflash or reinitialize only with proper tools and following manufacturer instructions.
If available, swap with a known-good identical module (or vehicle known-good harness) to confirm whether the module is faulty — only when legal and following anti-theft/registration/VTSS procedures.
After repairs, clear codes and perform a road or functional test to confirm communication is restored and the active aerodynamic components operate normally.

Likely causes

Damaged connector pins or corrosion at Active Aerodynamic Feature Control Module C
Module power (battery feed / fuse) is missing or intermittent
CAN bus shorted or open near module C
Module internal failure or faulty solder joints
Network termination missing or wrong resistance value