What this code means
B1648 may indicate that the body control module is detecting an open circuit on the front passenger’s door LF antenna line in the keyless access system.
What the vehicle may do
- Keyless access operation may be inconsistent around the front passenger door area.
- One-push start or smart key recognition can be affected depending on how the system is using the LF antenna signal.
Possible fault areas
- Possible open wiring or connector issues in the passenger door LF antenna circuit.
- Possible issue with the front passenger’s door LF antenna.
- Possible body control module side circuit concern.
Diagnostic path
Opening context
On this Civic, B1648 means the body control module may be seeing an open circuit on the front passenger’s door LF antenna line. In plain terms, the keyless access system may not be able to communicate through that passenger door antenna the way it expects. The driver may notice keyless entry or one-push start behavior that can be inconsistent, especially around that door area. The broad fault areas are the passenger door LF antenna circuit, the connectors and wiring between the antenna and the body control module, the antenna itself, or possibly the body control module side of the circuit. If other codes are present, check what they mean first so you do not chase this one out of order.
Start with the scan tool check
Before you get into the pinpoint checks, make sure you have reviewed the basic troubleshooting approach for this keyless access system. Then run the HDS self check. The scan tool path is One-Push Start, then Keyless Access Control Unit, then Self Check. For this code, the body control module sets the fault when it detects a disconnection in the front passenger’s door LF antenna line, so the first job is to separate a wiring problem from a module-side problem and then from the antenna itself.
Check the antenna circuit for an open
Next, do the open wire inspection for EXT_FR_AS+ and EXT_FR_AS-. Prepare by disconnecting body control module connector C, the 66P connector, and disconnecting the front passenger’s door LF antenna 8P connector. With both connectors disconnected and IG Mode OFF, check the EXT_FR_AS+ circuit. Normal is less than 1 Ω. Then check the EXT_FR_AS- circuit the same way, still with IG Mode OFF. Normal there is also less than 1 Ω. If either side is not normal, the problem is in the circuit, so stay with the wiring and connector path before moving on.
Check the body control module side
If the external circuit checks normal, move to the body control module inspection. This part of the path starts from the assumption that the outside circuit has already passed. Disconnect the front passenger’s door LF antenna 8P connector and perform the body control module inspection. With that antenna connector disconnected, do the module internal circuit inspection with IG Mode ON. The normal condition is voltage. There is also a note that this internal circuit is tied to a permanent power supply, so do not let the mode note pull you away from the stated test condition. If this inspection is abnormal, the fault points to the body control module side.
Judge the remaining antenna only after the other checks pass
Only after the circuit inspection and the module-side inspection show normal results do you call the remaining front passenger’s door LF antenna the problem. That order matters. Do not condemn the antenna until the two antenna wires have passed and the body control module inspection has not shown a fault.
Takeaway
The clean path for B1648 is simple: confirm the code with the keyless access self check, prove both LF antenna wires are not open, check the body control module side, and only then blame the passenger door LF antenna if everything else is normal. For more diagnostic training, visit stepdiagnostics.com.
Final check
Prove the antenna wiring first, then the module side, and only call the antenna after the other checks are normal.
For more guided automotive diagnostics, visit STEP Diagnostics.
