What this code means
B1668 may mean the body control module is detecting an open condition in the rear shelf low-frequency antenna circuit used by the keyless access system.
What the vehicle may do
- Keyless access operation may be inconsistent.
- One-push start behavior may be affected.
- The vehicle may store the code after the module detects the antenna line is disconnected.
Possible fault areas
- Possible rear shelf LF antenna concern.
- Possible open circuit or connector issue between the antenna and body control module.
- Possible body control module side circuit concern.
Diagnostic path
Opening context
On this Civic, B1668 points to the rear shelf LF antenna circuit being seen as open by the body control module. In plain terms, the module may not be seeing the low-frequency antenna circuit it expects for keyless access operation. The vehicle may have keyless access or one-push start behavior that feels inconsistent, and the possible fault areas can include the rear shelf LF antenna, the wiring between the antenna and the body control module, connector condition, or the body control module side of the circuit. If other codes are present, sort out that wider B-CAN context first so you do not chase this antenna circuit in isolation.
Confirm the code path
Start with the basic troubleshooting review, then confirm the fault with the HDS self check. In HDS, go into One-Push Start, then Keyless Access Control Unit, then run the Self Check. The logic behind this code is straightforward: if the body control module detects a disconnection on the rear shelf LF antenna line, it judges that circuit abnormal and stores B1668.
Check the antenna circuit wiring first
Next, inspect the EXT_M plus and EXT_M minus sides for an open circuit. With IG Mode OFF, disconnect the rear shelf LF antenna 2P connector and disconnect body control module connector C, the 66P connector. Then run the open-wire checks for EXT_M plus and EXT_M minus. Each side should measure less than 1 Ω. If either one is abnormal, treat that as a circuit problem and stay on the wiring and connector side before moving on.
Move to body control module inspection only after the external circuit passes
Once the external circuit checks normal, move to the body control module inspection. There is a note here worth keeping in mind: this body control module check can be performed before the other inspections because it includes the related inspection items, but if it fails, you still need to continue with the individual checks to determine which item actually failed. For this path, disconnect the rear shelf LF antenna 2P connector, set IG Mode ON, and perform the module internal circuit inspection. The normal result is listed as voltage. If that inspection is abnormal, the problem is on the body control module side.
Last remaining component in the path
If the wiring inspection is normal and the body control module inspection is normal, the remaining item in this diagnostic path is the rear shelf LF antenna. At that point, the conclusion is a problem with the rear shelf LF antenna.
Verification and takeaway
After the fault area is corrected, verify the repair and confirm B1668 stays gone with the same self-check approach. The key takeaway is to prove the antenna circuit first, prove the module side only after the external circuit is normal, and do not condemn the antenna until the other checks have passed. For more diagnostic training, visit stepdiagnostics.com.
Final check
B1668 is best approached by confirming the self check, proving both antenna circuit legs for continuity, then checking the body control module side before blaming the antenna.
For more guided automotive diagnostics, visit STEP Diagnostics.
