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Home / DTC / B1C90 — auxiliary relay system coil bulbs

B1C90 — auxiliary relay system coil bulbs

Detailed page for trouble code B1C90.

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22,461specific
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Code

B1C90

LAND ROVER B — Body

auxiliary relay system coil bulbs

Brand: LAND ROVER
Type: B — Body
Views: UK: 8 EN: 13 RU: 10
AI status
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Page language: EN

Causes

  • Failed auxiliary relay (stuck open, stuck closed, or open coil winding)
  • Open, shorted, corroded, or damaged wiring harness or connector at the relay or bulbs
  • Blown fuse or poor fuse contact in the auxiliary circuit
  • Faulty ground or power supply to the relay coil or bulbs
  • Failed bulb or shorted filament on the monitored auxiliary lighting/devices
  • Faulty body control module (BCM) or relay driver circuit

Symptoms

  • One or more auxiliary lights or accessories do not operate
  • Auxiliary function may operate intermittently
  • Warning/message on dash indicating auxiliary circuit fault
  • Related fuses blow repeatedly or relays click rapidly
  • Stored fault code(s) and possibly reduced functionality of affected systems

What to check

  • Retrieve freeze-frame data and related codes with a diagnostic scanner
  • Visually inspect auxiliary relays, relay sockets, bulbs, fuses, and wiring for damage, corrosion, or water ingress
  • Check condition and seating of connectors at the relay and bulbs
  • Verify fuse continuity and correct fuse rating
  • Measure battery voltage at the relay power feed with key on and engine running (typical 11.5–14.5 V)
  • Check for good ground at relay and component grounds (low resistance to chassis ground)

Signal parameters

  • Supply voltage to relay/power side: typical battery range 11.5–14.5 V
  • Relay coil resistance (typical 12 V automotive relays): approx. 50–200 ohms (manufacturer-specific)
  • Coil current when energized: roughly 60–250 mA (depends on relay coil resistance)
  • Control signal from BCM: either battery-side feed with switched ground or low-side drive; active output should show near 0 V to chassis ground when switched to ground, or battery voltage when switched to supply
  • Open-circuit coil reading: OL/infinite on ohmmeter
  • Short-to-ground on control feed: near 0 ohms to chassis ground

Diagnostic algorithm

  1. Connect a diagnostic scanner, read and record B1C90 and any related codes. Record freeze-frame and live data.
  2. Inspect the relay, relay socket, related fuse(s), and wiring for obvious damage, corrosion, looseness, or water ingress. Replace or repair as needed.
  3. Verify battery voltage at the relay power terminal with key on and engine running (expected ~11.5–14.5 V).
  4. With connector disconnected, measure coil resistance across the relay coil terminals. Compare to manufacturer spec or typical range (expect tens to a few hundred ohms). Replace relay if out of range or open.
  5. Check for continuity between the relay ground pin and chassis ground. Repair any high resistance or open grounds.
  6. With relay installed, perform an active test or energize coil using a fused 12 V supply (or use known-good relay). Observe if the relay clicks and the controlled circuit is powered. Monitor current draw for excessive draw (short) or no draw (open).
  7. Check downstream circuit: verify bulbs or auxiliary devices are functional by applying known good power/ground. Replace faulty bulbs or devices.
  8. Inspect and repair any damaged wiring (shorts to ground or battery, or open conductors). Use back-probing to monitor control signal from BCM while commanding the auxiliary function on/off.
  9. If wiring, relays, bulbs, and fuses check good but error persists, test BCM outputs and inputs per manufacturer procedures. Consider module programming or replacement only after wiring and components are confirmed good.
  10. Clear codes, operate the system under normal conditions, and confirm the fault does not return. Document repairs and re-scan.

Likely causes

  • Corroded connector or poor ground at the relay socket
  • Failed auxiliary relay coil
  • Blown fuse or open bulb in the monitored circuit
  • Damaged wiring harness causing open or short to battery/ground
  • Faulty BCM/relay driver (less common)

Fault status

⚠️ Status
Auxiliary relay coil/bulb circuit fault detected. Control module has logged B1C90 and may inhibit or disable the associated auxiliary function until the circuit fault is corrected.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.0 hours

Similar codes

B1C91 B1C92 B1C93 P0563 U0121
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