U1721

Code

Generic U — Network/User

Invalid Data Received from Control Module

Brand: Generic

Type: U — Network/User

Views: UK: 27 EN: 29 RU: 26

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Corrupted or noisy network communication (CAN/LIN)
Faulty or intermittent connector, wiring or ground to one or more modules
Failed or marginal CAN/LIN transceiver in a module
Module software mismatch or corrupted calibration/data
Duplicate node ID or misconfigured module on the bus
Low or unstable battery/charging system voltage during communication

Symptoms

Malfunction Indicator Lamp (MIL) or warning lights illuminated
Loss or intermittent loss of the function controlled by the origin module (e.g., ABS, transmission, instrument cluster)
Erratic or frozen gauge/cluster displays
Drivability symptoms if engine/ transmission messages are affected (hesitation, limp-home)
Modules not present or not responding on a network scan
Intermittent faults that clear when vehicle is restarted

What to check

Retrieve all stored U-codes and related P/C/B codes with a capable scan tool; note freeze frame and timestamp
Identify which module reported U1721 (source and requester) and list modules on the same network segment
Check battery voltage and charging system health (stable >12V during tests)
Visually inspect connectors, pins and wiring harnesses for damage, corrosion, mating issues and water ingress
Perform wiggle test on suspect wiring/connectors while monitoring DTCs and live data
Use a scan tool to monitor the specific message/frame and any related data parameters while reproducing the fault

Signal parameters

CAN bus idle (recessive): each line ~2.5 V, differential ~0 V
CAN dominant state: CAN_H ~3.5 V, CAN_L ~1.5 V, differential ~2.0 V (typical)
Termination: two 120 Ω resistors in parallel → ~60 Ω across CAN_H and CAN_L
Common CAN bit rates: 125 kb/s, 250 kb/s, 500 kb/s (verify vehicle-specific rate)
LIN bus idle: pulled up to battery voltage through resistor; active low when transmitting (module-specific timing and framing)
Expected message frequency and IDs are vehicle-specific—compare observed frame IDs and rates to OEM data

Diagnostic algorithm

Connect a capable scan tool and read all U-codes and related codes; record module names and time stamps.
Clear codes and attempt to reproduce while recording live data and messages; note if code returns immediately or only under certain conditions.
Confirm proper power and grounds to the module that reported the invalid data and to modules on the same network; repair any poor connections.
Visually inspect harnesses/connectors along the network for damage, water, corrosion, bent pins and repair as needed.
Check battery/charging system voltage during the fault; correct low/unstable voltage conditions before further testing.
Measure termination resistance across CAN_H and CAN_L (ignition off). If ≠ ~60 Ω, investigate missing/added termination resistors or wiring issues.
Use an oscilloscope or CAN/LIN bus analyzer to capture frames while reproducing: look for malformed frames, missing CRC, collisions, excessive noise or unexpected IDs.
Isolate the fault by disconnecting non-critical modules one at a time (with ignition off as required) to see if the error clears — observe network behavior after each change.
If a specific module is suspected, verify software/calibration part numbers and perform an update/reflash per OEM procedures; re-test.
If hardware fault suspected (bad transceiver or module), replace the module only after confirming wiring/power/grounds and re-test.
After repair, clear codes and perform a road/self-test to confirm stability and absence of U1721.
Note: follow vehicle manufacturer safety and module replacement/programming procedures to avoid unintended system issues.

Likely causes

Broken, chafed, or corroded network wiring or connector pins
Poor ground or power supply to the sending or receiving module
Failed CAN/LIN transceiver or module CPU producing invalid frames
Software mismatch after module replacement or incomplete reflash
Intermittent short to battery or ground affecting data integrity

Fault status

⚠️ Status

Invalid data received from control module. Message contents, checksum, timing, or ID did not meet expected parameters; communication integrity fault logged.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours