What this code means
B1664 may indicate the body control module is detecting an open condition on the rear interior LF antenna circuit.
What the vehicle may do
- The vehicle may have inconsistent keyless access behavior.
- One-push start operation can be affected if the key is not detected as expected inside the cabin.
Possible fault areas
- A possible rear interior LF antenna issue.
- Possible wiring or connector concerns in the antenna circuit.
- A possible body control module side concern.
Diagnostic path
Opening context
For B1664 on this Civic, think of the rear interior LF antenna circuit as the area under test. In plain terms, the body control module may be seeing that antenna line as disconnected or open. The customer may notice keyless access or one-push start behavior that seems inconsistent, because this antenna is part of how the vehicle detects the key inside the cabin. The broad possible fault areas are the rear interior LF antenna, its wiring and connectors, and the body control module side of the circuit. Before you get into the code-specific checks, start with the basic troubleshooting approach for this system. And if other codes are present, check what they mean first so you do not chase this circuit out of order.
Confirm the fault with the scan tool
Start by confirming the code with the HDS Self Check. On the HDS, go into One-Push Start, then Keyless Access Control Unit, then Self Check. That gives you the confirmation point before you start unplugging connectors and testing the rear interior antenna circuit.
Check the EXT_R circuits for an open
Next, check the EXT_R plus and EXT_R minus circuits for an open. With IG Mode OFF, disconnect the rear interior LF antenna 2P connector, and disconnect body control module connector C, the 66P connector. Then make the continuity checks at the disconnected female terminals for those two circuits. The normal result for EXT_R plus is less than 1 Ω, and the normal result for EXT_R minus is also less than 1 Ω. If either side is abnormal, the problem is in the circuit between the rear interior LF antenna and the body control module.
Check the body control module side
If the external circuit checks normal, move to the body control module inspection. This check can be done early, but if it fails, you still need to continue through the individual inspection items so you know exactly which item failed. For this check, disconnect the rear interior LF antenna 2P connector. With IG Mode ON, perform the voltage inspection at the disconnected rear interior LF antenna connector female terminals. If that inspection is abnormal, the fault area is the body control module.
When the other checks pass
If the circuit inspection is normal, and the body control module inspection is normal, the remaining item in this diagnostic path is the rear interior LF antenna. At that point, identify the rear interior LF antenna as the problem area. Do not jump there first; this conclusion only comes after the wiring side and the module-side inspection have both checked normal.
Closing takeaway
The clean way to handle B1664 is to confirm it with the HDS Self Check, prove the EXT_R plus and EXT_R minus circuits are not open, then separate a body control module-side issue from a rear interior LF antenna issue. Keep the testing in order and avoid guessing at terminals or parts. For more diagnostic training, visit stepdiagnostics.com.
Final check
Confirm the code, test the antenna circuit first, then separate module-side faults from the antenna itself.
For more guided automotive diagnostics, visit STEP Diagnostics.
