What does it mean?
B1959 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Rear MICU Lost Communication with Relay Control Module (RM Message) .
Symptoms
- Loads driven through the relay control module stop working, such as rear defogger, wipers or exterior lighting
- Several unrelated electrical functions fail together, which is the classic sign of a lost network member
- Warning lamps or messages in the cluster with no obvious mechanical fault
- Functions come back after an ignition cycle then fail again
- Additional lost communication codes stored in other modules
Likely causes
- Communication wire between the relay control module and the rear control unit open, shorted or corroded
- Relay control module without power because of a blown fuse, a failed main relay or a broken supply
- Ground fault shared by the relay control module, which upsets its signal reference
- Water intrusion into a fuse or relay box, a very common cause on this circuit
- Internal failure of the relay control module
- Unstable battery voltage, particularly during cranking, dropping modules off the network
- Rear control unit fault or damaged connector pins
How to fix it
- List which electrical loads have failed, the shared supply usually identifies the module involved
- Test the battery and charging system before chasing network faults
- Check every fuse and relay feeding the relay control module and inspect the box for water and corrosion
- Verify battery voltage and a clean ground at the relay control module connector with the ignition on
- Measure the communication wire between the two units for continuity and for shorts to ground or battery with both disconnected
- Wiggle test the harness at grommets and along the sill while watching for the code to reset
- Repair the supply, ground or wiring first, and replace the relay control module only when it stays silent with proven power, ground and wiring