What this code means
U0122 generally means the keyless access power control unit may have lost communication with the VSA modulator-control unit.
What the vehicle may do
- The vehicle may show warning lights or related system messages.
- Keyless access, power mode, or stability and brake-assist related behavior may be affected.
- The code may also be stored with only intermittent symptoms.
Possible fault areas
- Possible ignition power or fuse feed problem.
- Possible loose or poor connector contact.
- Possible open or high resistance in a power or communication wire.
- Possible F-CAN communication wiring issue between the involved modules.
- Possible control-unit fault after the power and communication wiring checks pass.
Diagnostic path
What U0122 Means Here
On this 2010 to 2015 Honda Civic, U0122 is a communication fault. In plain language, the keyless access power control unit may have lost communication with the VSA modulator-control unit. The vehicle may show warning lights, may have keyless access, power mode, or stability and brake-assist related symptoms, or it may only store the code intermittently. For this diagnostic path, stay focused on the basics: the VSA power feed, the B24 fuse circuit, the F-CAN wiring between the VSA modulator-control unit and the keyless access control unit, connector condition, and only after those checks pass, the control unit itself.
Verify the Code First
Start by proving the fault is active. Clear the DTCs with the HDS. Press the engine start/stop button to select OFF mode, then select ON mode. Wait for at least 6 seconds, then check for DTCs again with the HDS. If U0122 is indicated again, continue with the fuse check. If U0122 is not indicated, treat it as an intermittent failure for now. The system is OK at this time, so look closely for loose or poor connections before going any deeper.
Check the VSA Power Fuse
Next, check Fuse No. B24, rated at 7.5 A, in the under-dash fuse and relay box. If the fuse is OK, move on to the IG1_VSA/ABS power feed check. If the fuse is not OK, replace the fuse. If that fuse blows again, repair a short to ground in the No. B24 (7.5 A) fuse circuit.
Check the IG1_VSA/ABS Feed for an Open
Now check the ignition feed to the VSA modulator-control unit. Select OFF mode, then disconnect the VSA modulator-control unit 22P connector. Select ON mode and measure voltage on the IG1_VSA/ABS circuit between VSA modulator-control unit 22P connector terminal No. 8, the gray wire, and body ground. You are looking for battery voltage. If battery voltage is present, the IG1_VSA/ABS wire is OK, so continue to the F-CAN continuity check. If battery voltage is not present, repair an open or high resistance in that wire.
Check the F-CAN Lines Between the Modules
With OFF mode selected, disconnect keyless access control unit connector C, the 24P connector. At this point the VSA modulator-control unit 22P connector remains disconnected. Check continuity on both F-CAN lines. For F-CAN-H, check between keyless access control unit connector C terminal No. 4, the white wire, and VSA modulator-control unit 22P connector terminal No. 10, the white wire. For F-CAN-L, check between keyless access control unit connector C terminal No. 5, the red wire, and VSA modulator-control unit 22P connector terminal No. 15, the red wire. If continuity is present on the F-CAN wires, those wires are OK, and the next action is to replace the keyless access control unit. If continuity is not present, there is an open or poor connection in the F-CAN wiring between the VSA modulator-control unit and the keyless access control unit.
Verify the Repair and Close
Keep verification separate from the circuit testing. Once the wiring, fuse circuit, or control unit path has been handled, use the same code check from the beginning: clear the DTCs with the HDS, switch OFF mode and then ON mode, wait for at least 6 seconds, and confirm U0122 does not return. The takeaway is simple: prove the code, confirm the VSA power feed, prove the F-CAN lines, and only then make the control unit call. For more diagnostic training, visit stepdiagnostics.com.
Final check
Prove the code first, then work through power feed and communication continuity before making a module decision.
For more guided automotive diagnostics, visit STEP Diagnostics.
