What this code means
B1648 may mean the keyless access control logic is seeing a possible disconnection in the front passenger's door LF antenna line.
What the vehicle may do
- Keyless access operation may be inconsistent.
- One-push start behavior may be affected in some situations.
- The concern may be more noticeable around the front passenger side of the vehicle.
Possible fault areas
- Possible open or connection issue in the front passenger's door LF antenna circuit.
- Possible connector or terminal condition between the antenna and body control module.
- Possible body control module side circuit concern.
- Possible front passenger's door LF antenna concern.
Diagnostic path
Opening context
On this Civic, B1648 is tied to the keyless access system. In plain terms, the body control module may be seeing the front passenger's door LF antenna line as disconnected. The driver may notice keyless access or one-push start behavior that feels inconsistent, especially around that side of the vehicle. The broad possible fault areas are the antenna circuit, the connector path between the antenna and body control module, the body control module side of the circuit, or the front passenger's door LF antenna itself. Before you get deep into pinpoint testing, start with the basic system checks and follow a structured diagnostic approach. If other codes are present, check what they mean first and do not chase this one in isolation.
Confirm the code with the scan tool
Start the actual B1648 path with the HDS Self Check. Use this path: One-Push Start, Keyless Access Control Unit, then Self Check. This gives you the control-unit view of the fault before you start unplugging connectors or loading the circuit.
Check the antenna circuit for an open
Next is the EXT_FR_AS+ and EXT_FR_AS- open wire inspection. With IG Mode OFF, disconnect body control module connector C (66P), and disconnect the front passenger's door LF antenna 8P connector. The check is made at the disconnected female terminals on those connectors. Make sure the correct terminal locations are positively identified before probing; do not guess at terminal positions. For the EXT_FR_AS+ open wire inspection OFF, normal is less than 1 Ω. For the EXT_FR_AS- open wire inspection OFF, normal is also less than 1 Ω. If either result is abnormal, the fault is in the circuit.
Check the body control module side
Move to the body control module inspection only with the external circuit normal. There is a note that this inspection can be run before the individual checks because it includes the inspection items, but if it fails, keep going through the individual checks so you know what actually failed. For this inspection, disconnect the front passenger's door LF antenna 8P connector. With IG Mode ON, check the module internal circuit at the disconnected front passenger's door LF antenna connector female terminals. The normal condition listed for the module internal circuit inspection ON is Voltage. The note also points out the permanent power supply, so be careful not to over-read vehicle mode by itself. If this result is abnormal, there is a problem with the body control module.
Decide what is left after the tests
If the circuit inspection is normal and the body control module inspection is normal, the remaining item in this diagnostic path is the front passenger's door LF antenna. At that point, the fault is identified as a problem with that antenna, not the wiring path or module side already checked.
Verification and takeaway
Keep verification separate from the pinpoint checks. After the actual fault has been corrected, use the HDS Self Check path again and confirm B1648 is no longer being reported. The takeaway is simple: prove the antenna circuit first, prove the body control module side second, and only then call the front passenger's door LF antenna the remaining fault. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnosis works by separating the antenna wiring path, the body control module side, and the antenna itself before making a call.
For more guided automotive diagnostics, visit STEP Diagnostics.
