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Home / DTC / B1085 — R.Def.output circuit short 12V

B1085 — R.Def.output circuit short 12V

Detailed page for trouble code B1085.

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Code

B1085

MITSUBISHI B — Body

R.Def.output circuit short 12V

Brand: MITSUBISHI
Type: B — Body
AI status
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ready
Completed 100%
Page language: EN

Causes

  • Damaged or chafed wiring contacting battery positive or fused B+
  • Shorted connector (pin to B+ or corrosion bridging terminals)
  • Stuck or welded defogger relay contacts or faulty relay
  • Failed body control module (BCM) or defogger controller output stage
  • Aftermarket accessory tied into rear circuit causing backfeed
  • Blown or incorrectly installed fuse or fusible link

Symptoms

  • Rear defogger operating continuously or not responding to switch
  • Defogger indicator lamp stays on or behaves erratically
  • Battery drain when vehicle is off (possible parasitic draw)
  • Related fuse may blow or show signs of overheating
  • Diagnostic trouble code B1085 stored and possibly other body codes

What to check

  • Visual inspection of rear defogger wiring, connectors, relay and fuse for damage or corrosion
  • Check fuse(s) and relay for the defogger circuit; swap relay with known-good of same type if available
  • With ignition off, measure resistance between suspected output pin and battery positive — should be high (open)
  • Back-probe output pin with ignition off/on and while operating defogger switch to observe voltage behavior
  • Disconnect rear defogger connector (or relay output) and see whether code clears / behavior changes
  • Check for aftermarket accessory splices at or near trunk/hatch wiring

Signal parameters

  • Battery voltage (typical): 12.0–14.5 V
  • Expected output voltage with defogger OFF: open or ~0 V (no B+ present on output pin)
  • Expected output voltage with defogger ON: battery voltage (~12–14.5 V) present at grid/element via relay
  • If B1085 present: output pin measures battery voltage with switch OFF (unexpected)
  • Resistance between output pin and battery positive with circuit isolated: should be very high / infinite if no short

Diagnostic algorithm

  1. Retrieve freeze frame and confirm B1085 is current or historic. Clear codes and attempt to re-run to confirm repeatability.
  2. Visually inspect wiring and connectors from BCM/relay to rear window grid and harness routing through the hatch and bumper areas for chafing, pinching or signs of heat/corrosion.
  3. Inspect defogger fuse(s) and relay. Replace relay with known-good unit and re-test. Check socket for corrosion or melted terminals.
  4. With ignition OFF, disconnect the rear defogger connector (or unplug the relay output). Measure resistance between the vehicle battery positive and the control output pin; a low resistance indicates a direct short to B+ in wiring or load.
  5. With the connector disconnected, clear the code and actuate the defogger. If the code does not return, the short is in the downstream wiring or load (grid/connector). If the code remains with connector disconnected, suspect BCM/module output.
  6. Back-probe the control output at the module/relay with ignition ON and operate defogger switch: verify expected behavior (OFF = open/0 V, ON = B+). If OFF shows B+, trace wiring for backfeed or internal module fault.
  7. Isolate wiring sections by disconnecting intermediate connectors to locate the short. Use a fused jumper or current-limited bench supply when performing resistance/short checks to avoid damage.
  8. Inspect rear window heater grid for damage or repaired splice that may contact B+ or body. Repair or replace damaged grid sections and connectors.
  9. If wiring and load are good but output still shows B+ when it should not, replace or repair the BCM/defogger controller output following manufacturer procedures.
  10. After repair, clear codes and verify operation through several cycles; confirm no repeat DTC and proper voltage behavior.

Likely causes

  • Pinched/chafed wiring harness in rear harness near hatch or bumper contacting battery feed
  • Corroded or damaged connector at defogger relay/module or rear window connector
  • Relay stuck closed or welded causing constant B+ on output
  • BCM output transistor failed, sourcing voltage internally
  • Aftermarket audio/security wiring tied into same circuit creating backfeed

Fault status

⚠️ Status
Rear defogger output circuit short to 12V detected — control unit senses battery voltage on the R.Def output when it should be open or low.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-2.5 hours

Similar codes

B1084 B1086 B1090 B1100 B1140
406

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