What does it mean?
B1721 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Mirror Left and Right Control Switch Circuit .
Symptoms
- Mirror glass no longer swivels left or right, while up and down still works
- Both mirrors dead on the horizontal axis if the shared control line is at fault
- Mirror motor buzzes without the glass moving
- Memory or reverse-tilt mirror function no longer positions the glass correctly
- Fault appears mainly in wet weather or after the door has been slammed
Likely causes
- Dirty or burnt contacts on the left/right side of the mirror joystick
- Moisture inside the switch pack from a window left open or a leaking door seal
- Chafed or broken horizontal signal wire in the door-to-body loom boot
- Green corrosion on the door connector pins carrying the mirror signals
- Short between the horizontal and vertical control wires where the loom is crushed behind the trim panel
- Faulty mirror motor drawing abnormal current and dragging the control line down
- Door module input stage failure
How to fix it
- Read the codes and watch the horizontal switch value while sweeping the joystick left and right
- Inspect and clean the switch pack contacts; look for water marks inside the housing
- Test with a known good switch pack if available to separate switch from wiring
- Flex the loom inside the door boot while monitoring the live value for dropouts
- Measure continuity of the horizontal control wires and check for a short between them and the vertical pair
- Check current draw of the mirror motor; a seized motor can pull the circuit out of range
- Repair the loom or replace the switch, mirror actuator or door module as the tests dictate