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Home / DTC / C121D08 — Assistant Side Indicator Circuit Failure

C121D08 — Assistant Side Indicator Circuit Failure

Detailed page for trouble code C121D08.

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Code

C121D08

HYUNDAI C — Chassis

Assistant Side Indicator Circuit Failure

Brand: HYUNDAI
Type: C — Chassis
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Burned or failed bulb/LED module
  • Open or shorted wiring in the indicator circuit
  • Corroded or loose connector at lamp or harness
  • Poor or missing ground connection
  • Faulty lamp socket or module
  • Blown fuse or failed relay (if applicable)

Symptoms

  • Passenger-side exterior indicator (turn signal or side indicator) does not light
  • Turn signal/hazard hyper-flash (very fast flashing) when the affected side is used
  • Dash indicator may show warning or lamp-out message
  • Partial illumination or intermittent operation of the indicator
  • Related ADAS/side-assist warnings if lamp is used by those systems

What to check

  • Read stored DTCs and freeze-frame data with scan tool; note any accompanying codes
  • Visually inspect the affected lamp, socket, and wiring for damage, corrosion, or water ingress
  • Operate turn signal/hazard while observing lamp and listening for relay or clicker
  • Check fuses and relays related to exterior lighting
  • Perform wiggle test on harness while operating the indicator to reproduce fault
  • Measure supply and ground at the lamp connector with the lamp commanded ON (use multimeter or scope)

Signal parameters

  • Expected supply voltage at lamp (lamp ON): ~11–14 V (vehicle battery nominal)
  • Open-circuit: OL/infinite ohms between supply and lamp (bulb removed)
  • Incandescent lamp current: typically 0.5–2.0 A (depending on bulb rating)
  • LED lamp current: typically 0.02–0.5 A (low current) — depends on module and resistors/drivers
  • If LED: control may be PWM; typical PWM frequency range 100–400 Hz, duty cycle varies with command
  • Ground resistance: low, typically

Diagnostic algorithm

  1. Retrieve and record DTC(s) and freeze frame data; clear code and re-test to verify reproducibility.
  2. Visually inspect the lamp assembly, lens, and connector for corrosion, moisture, or physical damage; repair as needed.
  3. With the indicator demand active, measure voltage at the lamp power pin; verify ~11–14 V when on.
  4. Measure ground continuity from lamp ground pin to chassis; repair any high-resistance ground.
  5. If incandescent lamp, remove and bench-test bulb or replace with known-good bulb; for LED, substitute known-good module if available.
  6. If lamp and connector appear good, inspect harness for opens/shorts/backfeeds; perform continuity and resistance checks to BCM connector.
  7. Wiggle test harness while operating lamp to try to reproduce intermittent faults; locate and repair wiring chafe or broken conductors.
  8. Check related fuses/relays and BCM connector pins for damage; measure at BCM pin to confirm supply/ground at module.
  9. If wiring and lamp are good but fault persists, consult wiring diagrams and consider BCM software update or replacement following manufacturer procedures.
  10. After repair, erase codes and confirm normal operation under multiple cycles and verify no reoccurrence.

Likely causes

  • Failed lamp (bulb or LED)
  • Open circuit or disconnected connector at the lamp
  • Corroded/poor ground at lamp or harness
  • Damaged wiring harness (pinched/chafed) near door or bumper
  • Contaminated or water-damaged lamp connector

Fault status

⚠️ Status
BCM detected abnormal condition on assistant-side indicator circuit (open, short, or abnormal load) while commanded ON. Lamp may be inoperative or intermittent.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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