What this code means
U0122 often means the PCM may have a communication problem with the VSA modulator-control unit on the vehicle network.
What the vehicle may do
- Brake, ABS, or stability-control warning indicators may stay on.
- Vehicle stability or traction-related functions can be limited while the fault is active.
- The code may be current or intermittent depending on connector, wiring, communication, or module conditions.
Possible fault areas
- Possible network wiring or connector issues between the PCM and VSA modulator-control unit.
- Possible loose terminals or poor connections at the involved control units.
- Possible VSA modulator-control unit software or module communication issue after the wiring path checks good.
Diagnostic path
Opening context and setup
On this Accord, U0122 is a communication fault between the PCM and the VSA modulator-control unit on the F-CAN network. In plain terms, the engine controller may not be getting the VSA messages it expects. The vehicle may have brake, ABS, or stability-control indicators stay on, and the fault area can be a possible network issue, connector or terminal problem, module communication problem, or software-related issue. This information applies to the 2.4L four-cylinder Accord application identified as 2017 Honda Accord Sedan L4-2.4L (K24W1). Before troubleshooting, save the captured data and any on-board snapshot, then review the general troubleshooting information. Keep the diagnosis in order so you are not chasing a network fault from the wrong end.
Initial DTC check
Turn the ignition switch to ON (II), or select ON mode with the engine start/stop button. Use the HDS to check for Pending or Confirmed DTCs. If U0122 is setting at the same time as the related F-CAN network code, handle that network code first. If U0122 is not setting with that related code, continue with the U0122 path.
Duplicate the fault
Next, clear the DTC with the HDS, then check again for Pending or Confirmed DTCs. If U0122 comes right back, the failure is duplicated and you can continue. If U0122 does not return, treat it as an intermittent condition. At that point, the system is OK at this time, and the next move is to check for poor connections or loose terminals at the involved control-unit connectors. If the path is not clear, pause, recheck the earlier diagnostic direction, and do not guess.
Check VSA communication
With the fault duplicated, check whether the HDS can communicate with the VSA system. If the HDS communicates with the VSA modulator-control unit, move into the F-CAN wire checks. If the HDS cannot communicate with the VSA modulator-control unit, do not keep going down this U0122 wire-check path yet. Move to the symptom path for the ABS, brake system, and VSA indicators staying on.
Check the F-CAN-H line for an open
For the high-side network wire, turn the ignition switch to LOCK (0), or select OFF mode. Jump the SCS line with the HDS. Disconnect PCM connector A (51P), and disconnect the VSA modulator-control unit connector used for this test. The continuity check is on the F CAN-H circuit from the VSA modulator-control unit 21P connector No. 8, WHT, to PCM connector A (51P) No. 2, WHT. If there is continuity, the F CAN-H wire is OK and you move to the low-side wire. If there is no continuity, repair the open in the F CAN-H wire between PCM A2 and the VSA modulator-control unit, then verify the repair.
Check the F-CAN-L line and module path
Now check continuity on the F CAN-L circuit. The test condition stays the same: IG LOCK (0) or OFF mode, PCM connector A (51P) disconnected, and the VSA modulator-control unit 21P connector disconnected. Check from the VSA modulator-control unit 21P connector No. 15, RED, to PCM connector A (51P) No. 1, RED. If there is no continuity, repair the open in the F CAN-L wire between PCM A1 and the VSA modulator-control unit, then verify the repair. If there is continuity, the F CAN-L wire is OK. At that point, update the VSA modulator-control unit if it does not have the latest software, then verify the repair. If U0122 is still indicated after the update, replace the original VSA modulator-control unit, then verify the repair again.
Verify the repair
Keep verification separate from the wire testing. Reconnect all connectors, exit SCS mode with the HDS, then turn the ignition switch to ON (II), or select ON mode. Reset the PCM with the HDS, do the PCM idle learn procedure, and check again for Pending or Confirmed DTCs. If U0122 is still indicated, check for poor connections or loose terminals at the VSA modulator-control unit and the PCM, then go back to the first DTC check and work the path again. If U0122 is not indicated, troubleshooting for this code is complete. If any other Pending or Confirmed DTCs are present, check what they mean next. Bottom line: prove the code is current, prove communication to VSA, test both F-CAN wires for opens, then handle software or the VSA modulator-control unit only after the wiring path passes. For more diagnostic training, visit stepdiagnostics.com.
Final check
Diagnose U0122 by confirming the code, checking VSA communication, testing the F-CAN high and low circuits for opens, and keeping repair verification separate from the circuit checks.
For more guided automotive diagnostics, visit STEP Diagnostics.
