C1721: OBD-II code C1721 flags the left rear damper actuator circuit shorted to ground — the RSS module sees a dead short pulling the drive line to ground.

Symptoms

• The 'Service Ride Control' warning is displayed and left rear damping reverts to a default fixed level.
• The left rear corner may feel harsher or softer than the others because adaptive damping is inactive.
• The RSS module may disable adaptive control on all four corners as a safety fallback while the fault is active.

Common causes

• A chafed left rear damper actuator wire touching a chassis ground point, often on a suspension bracket or wheel-well liner.
• An internally shorted damper actuator coil pulling the drive circuit to ground.
• Water intrusion into the left rear damper connector causing a short from the drive wire to a ground pin.

What to do?

Disconnect the left rear damper actuator and inspect the harness for chafing against chassis metal. Verify no continuity from the drive wire to ground with the actuator unplugged. Measure actuator resistance — most read 4-10 Ω when good. Replace the actuator or repair the harness as needed.