U0421 indicates that a controller (usually the PCM or body control module) is receiving invalid or missing data from the suspension control module over the vehicle's communication network. This points to a communication or module fault rather than an engine performance issue.
The module itself may have internal electronics failure, causing it to stop transmitting valid data on the bus.
Damaged, corroded, or loose pins in the harness connecting the suspension control module to the CAN bus can corrupt or interrupt messages.
Faults elsewhere on the bus (shorted wires, terminating resistor issues, or another faulty module) can prevent the suspension module's messages from being received correctly.
Incorrect voltage or a poor ground at the suspension control module can cause it to reset or transmit garbled data.
The suspension control module's firmware may be corrupted or outdated, leading to invalid message formatting.
While the PCM or body control module could theoretically fail to interpret valid data, this is uncommon and should be considered only after all other causes are ruled out.
No. The PCM is rarely the cause of U0421; the issue is almost always in the suspension control module, its wiring, or the CAN bus. Replacing the PCM without addressing those components is unlikely to resolve the code.
A blown fuse that removes power or ground from the suspension control module can lead to missing or invalid messages, which may trigger U0421. Checking fuses is part of the power/ground test.
Clearing the code only turns off the warning light; if the underlying communication fault remains, the code will return shortly after driving.
Driving is generally possible, but active suspension functions may be disabled, affecting ride comfort and handling. It is advisable to diagnose and repair the issue promptly.