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Home / DTC / C1704 — Fail Safe Relay - Failure | ECU Fail-Safe Relay Fail | EPS Module(fail safe relay stuck) Malfunction | st Module(Fail Safe Relay Stuck)

C1704 — Fail Safe Relay - Failure | ECU Fail-Safe Relay Fail | EPS Module(fail safe relay stuck) Malfunction | st Module(Fail Safe Relay Stuck)

Detailed page for trouble code C1704.

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C1704

HYUNDAI C — Chassis

Fail Safe Relay - Failure | ECU Fail-Safe Relay Fail | EPS Module(fail safe relay stuck) Malfunction | st Module(Fail Safe Relay Stuck)

Brand: HYUNDAI
Type: C — Chassis
AI status
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Page language: EN

Causes

  • Fail‑safe relay contacts stuck (mechanical)
  • Internal fail‑safe relay coil or control circuit failure inside the EPS module
  • Blown fuse or insufficient battery/supply voltage to relay
  • Corroded/loose connector or damaged wiring (open, short to ground or +B) to relay
  • Water intrusion or contamination of relay/connectors
  • Intermittent power or ground due to poor battery connections or charging system issues

Symptoms

  • EPS warning lamp or steering assist warning displayed
  • Reduced or loss of electric power steering assist (heavy manual steering)
  • Steering may switch to a default fail‑safe mode with limited assist
  • Possible multiple EPS‑related DTCs stored
  • Intermittent assist or assist lost during certain conditions (engine off/on, vehicle start)

What to check

  • Use a scan tool to read DTCs and freeze frame data; note event conditions (battery voltage, engine rpm)
  • Check battery voltage and charging system (resting and cranking voltage)
  • Visually inspect EPS module, relay area, connectors and wiring for corrosion, water ingress, damage, or loose pins
  • Check fuses related to EPS and fail‑safe relay (replace if blown)
  • Command/follow relay activation using a capable scan tool or perform an in‑vehicle relay actuation test if supported
  • Measure supply voltage at the relay power terminal (+B) and at the relay output when commanded

Signal parameters

  • Relay control signal (from EPS control) — expected: 0 V (off) or battery voltage/drive pulse when commanded
  • Relay coil voltage — expected ≈12 V when energized (subject to vehicle nominal battery voltage)
  • Supply (+B) at relay input — expected: battery voltage (approx. 12–14 V)
  • Relay output to EPS unit — expected: near battery voltage when relay closed, open/OL when relay open
  • Ground continuity — expected low resistance (
  • Relay contact resistance — expected low (≤0.5 Ω) when closed; high/OL when open

Diagnostic algorithm

  1. Retrieve all stored DTCs and freeze frame; record VIN, software level and any related codes.
  2. Check and record battery resting voltage and charging voltage; ensure battery ≥12.4 V and charging system operational.
  3. Visually inspect EPS module, fail‑safe relay area, harness and connectors for water intrusion, corrosion, bent pins or damage. Repair as needed.
  4. Inspect and test fuses and fusible links for EPS power circuits; replace blown fuses and retest.
  5. With connector(s) disconnected, check continuity and resistance of wiring from EPS relay connector to power and ground. Repair any opens/shorts.
  6. Command the fail‑safe relay ON/OFF using a scan tool if supported; verify relay clicks and that voltage appears/disappears at output terminal. If relay is internal to EPS and cannot be commanded externally, record module behavior.
  7. Measure relay coil/control pin while commanding: confirm control driver changes (0 V to near battery or PWM). If the control driver does not change, suspect EPS module drive circuit fault.
  8. If relay is an external replaceable unit, swap with known good relay or bench test relay. If internal to EPS and tests indicate internal relay failure, replacement of EPS assembly is usually required.
  9. After repairs, clear codes, perform functional test/driving check and monitor for recurrence. If code returns, escalate to module replacement or bench repair per manufacturer service manual.

Likely causes

  • Internal fail‑safe relay stuck closed or open inside EPS module
  • Open or shorted relay coil circuit (wiring/connector)
  • Supply fuse blown or battery voltage low causing relay to not energize
  • Connector corrosion/water ingress at EPS relay connector
  • EPS module hardware fault causing incorrect drive of relay

Fault status

⚠️ Status
EPS module reports Fail‑Safe Relay Stuck/Failure — relay circuit not switching as commanded; EPS entered fail‑safe mode. Stored when relay control or contact state is inconsistent with expected operation.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-2.5 hours

Similar codes

C1700 C1701 C1702 C1600 U0416
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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