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Home / DTC / B2785 — Accessory Power Relay Circuit Malfunction (Body Control Module)

B2785 — Accessory Power Relay Circuit Malfunction (Body Control Module)

Detailed page for trouble code B2785.

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Code

B2785

Generic B — Body

Accessory Power Relay Circuit Malfunction (Body Control Module)

Brand: Generic
Type: B — Body
Views: UK: 2 EN: 4 RU: 7
AI status
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Page language: EN

Causes

  • Blown or intermittent fuse for accessory power feed
  • Faulty accessory power relay (stuck open/closed or coil short)
  • Open, short to ground, or short to voltage in relay control or load wiring
  • Poor ground or corroded/loose connector at relay, fuse, or accessory
  • Water intrusion or corrosion in BCM connector or relay socket
  • Faulty BCM output driver or internal BCM electronics

Symptoms

  • No power to accessory outlets (12V sockets), cigarette lighter, or some convenience circuits
  • Accessories work intermittently or only when tapping relay/connector
  • Fuses for accessory circuits may blow repeatedly
  • Battery drain or parasitic draw when accessory circuit stuck on
  • Related BCM warnings or communication notices on scan tool

What to check

  • Scan BCM for B2785 and related codes; record freeze frame/conditions
  • Visually inspect fuses, relay, relay socket, and connectors for corrosion or damage
  • Check for aftermarket accessories tied into accessory power circuits
  • Verify battery voltage is within nominal range (12.4–14.8 V) during tests
  • Perform wiggle test at relay/connector while monitoring circuit operation

Signal parameters

  • Relay coil control: 0 V (off) or battery voltage (~12 V) when BCM energizes relay
  • Output/load terminal: battery voltage (~12 V) when relay closed, near 0 V when open
  • Coil resistance: typical relay coils often ~50–200 ohms (compare to known-good spec)
  • Ground continuity:
  • BCM command: digital on/off command present on scan tool (if supported)
  • Current draw: accessory circuits should not exceed expected load; watch for parasitic draw if stuck on

Diagnostic algorithm

  1. Retrieve freeze frame and related DTCs using an OBD scan tool; note conditions when fault set.
  2. Visually inspect accessory power fuse(s), relay, relay socket, BCM connector, and wiring for corrosion, heat damage, or loose terminals. Repair as needed.
  3. With key OFF, check fuse continuity and proper fuse seating. Replace any blown fuses and retest to see if code returns.
  4. Probe relay socket: check for battery voltage on supply terminal, ground on ground terminal, and control voltage on coil terminal while commanding accessory ON via key or scan tool. Compare to expected values.
  5. If relay control does not get a command, backprobe BCM output pin and check for voltage when accessory is requested. If no command, verify BCM power/ground and bus communications.
  6. Swap in a known-good relay (same part number) and retest circuit operation to rule out relay coil/contact failure.
  7. If supply or control wiring suspect, perform continuity and short-to-ground/short-to-voltage tests between relay socket, fuse, accessory devices, and BCM. Repair any shorts or opens found.
  8. If wiring and relay test good but BCM does not command or shows abnormal driver voltages, verify BCM programming and power/ground. Consider reprogramming or replacement only after confirming external wiring and loads are good.
  9. After repairs, clear codes and perform functional tests and road or runtime checks to confirm code does not return. Monitor for parasitic drain if circuit previously stuck on.

Likely causes

  • Blown fuse or loose fuse connection on the accessory feed
  • Corroded or loose relay socket/connector causing intermittent contact
  • Wiring short to ground on relay control or load circuit
  • Failed accessory power relay
  • Internal BCM driver failure (less common)

Fault status

⚠️ Status
Accessory Power Relay Circuit Malfunction detected by BCM. Check fuse, relay, wiring, connectors, and BCM output driver.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-2.5 hours

Similar codes

B2780 B2781 B2786 P0562 P0563 U0121
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