U0256

Code

Generic U — Network/User

Lost Communication With Front Controls Interface Module A

Brand: Generic

Type: U — Network/User

Views: UK: 31 EN: 90 RU: 48

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Page language: EN

Causes

Faulty Front Controls Interface Module A (internal failure)
Power supply or ground fault to the FCIM (open, high resistance, intermittent)
Damaged, corroded, or disconnected connector at the FCIM
Open, shorted, or high-resistance CAN bus wiring (CAN High/CAN Low)
Bus termination problem (missing or wrong resistance)
Short to battery or ground on CAN wires

Symptoms

MIL or network warning lamp illuminated
Loss of functions controlled by FCIM A (e.g., HVAC controls, front switches, interior lighting, infotainment front controls)
Related modules reporting multiple U-codes or communication errors
Intermittent or permanent loss of front control responsiveness
Certain convenience features inoperative (e.g., window/door/seat controls)
No FCIM module listed or module not responding on a diagnostic scanner

What to check

Read and record all manufacturer and generic codes and freeze frame data with a scan tool
Check for additional network communication codes from other modules
Visually inspect FCIM connector and wiring for corrosion, damage, or water intrusion
Verify battery voltage and inspect related fuses and relay supplying FCIM (with key ON and engine OFF)
Verify good chassis and battery grounds for the FCIM (measure voltage drop under load)
Scan for presence of FCIM on the bus (module enumerates on the network)

Signal parameters

CAN bus idle voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (recessive)
CAN dominant state voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (approx.)
Typical CAN bit rates: 250 kb/s or 500 kb/s (verify manufacturer spec)
Termination resistance across CAN_H to CAN_L ≈ 60 ohms (two 120 Ω terminators in parallel)
FCIM supply: 12 V battery (VBAT) and ignition-switched 12 V (verify vehicle-specific pins)
Ground continuity to chassis near 0 ohms

Diagnostic algorithm

Confirm U0256 is active and note any accompanying codes. Attempt to communicate with FCIM using a capable scan tool. If the module is not present on the bus, proceed.
Verify battery voltage and inspect/replace any fuses or relays that supply the FCIM. Clear codes and re-check.
With ignition ON, measure DC voltage at FCIM power and ground pins. Repair any power or ground faults. Re-scan for the code.
Visually inspect FCIM connector and harness for corrosion, bent pins, water intrusion or damage. Repair or replace damaged connector/harness as needed.
Check CAN bus presence: measure idle voltages on CAN_H and CAN_L at the FCIM connector. If voltages are out of expected range, trace wiring to nearest junction, looking for open/short.
Measure termination resistance across CAN_H to CAN_L at key points. If termination not correct (~60 Ω), locate missing/shorted terminator and repair.
Use an oscilloscope or CAN bus analyzer to capture waveform while attempting communication. Look for dominant/recessive transitions, noise, or stuck bus. Isolate by disconnecting nearby modules one at a time and re-testing.
If wiring and bus are good, check for software updates or reprogramming requirements for FCIM after service. Reprogram or reflash per manufacturer procedures if needed.
If FCIM does not respond after verifying power, ground, and correct bus signals, replace the module and program/configure per manufacturer instructions. Perform final verification: clear codes, exercise functions, and perform road or functional test.
Document repairs and re-check for recurrence. If intermittent persists, perform extended monitoring or consult manufacturer technical service bulletins.