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DTC codes / B08xx · Airbags, seats, doors, lights and comfort systems / B0851

B0851Right Brake Lamp Circuit

B0851 indicates that the body control module (BCM) has detected a fault in the right brake lamp circuit, which includes the wiring, bulb, and connections for the passenger-side brake light. The BCM monitors the circuit for expected voltage or current when the brake pedal is pressed and sets this code when the signal is outside the normal range.

SystemBody
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaAirbags, seats, doors, lights and comfort systems

Symptoms you may notice

  • The right brake light does not illuminate when the brake pedal is pressed.
  • The brake light warning indicator on the dashboard may be illuminated.
  • The right brake light may stay on continuously, even when the pedal is released.
  • The turn signal or hazard lights on the right side may not function if the same bulb is used for both functions.
  • Often no symptoms besides the warning light, especially if the faulty circuit is only intermittently detected.

Common causes, most likely first

  1. Burnt-out or damaged right brake light bulb.
  2. Corroded, loose, or damaged bulb socket or connector.
  3. Broken, chafed, or open wire in the right brake lamp circuit.
  4. Failed brake light switch (though this usually affects both sides, it can set a code for one side if the BCM logic is sensitive).
  5. Internal fault in the body control module (BCM) or rear electronic module.
  6. Aftermarket LED bulb or incorrect bulb type causing a resistance mismatch.

How to diagnose and fix it

  1. Check the right brake light bulb and replace it if burnt out or damaged.
  2. Inspect the bulb socket and connector for corrosion, tightness, and proper contact; clean or repair as needed.
  3. Test the right brake lamp circuit wiring for continuity and shorts using a multimeter, following the manufacturer's wiring diagram.
  4. Verify the brake light switch operation and signal to the BCM, especially if both brake lights are affected.
  5. Check for aftermarket accessories or LED bulbs that may cause resistance issues, and restore to OEM specification if necessary.
  6. If all external components test good, diagnose the BCM or rear electronic module for internal faults, which may require professional scan tool testing.

Common Missteps in Right Brake Lamp Diagnosis

  • A faulty brake light switch often sets codes for both left and right circuits, so a code only for the right side usually points to a problem in that specific circuit rather than the switch.
  • Aftermarket LED bulbs can trigger this code because their lower current draw may fall outside the BCM's expected range, even though the bulb lights up normally.
  • The right brake lamp circuit may share a ground point with other rear lighting components, so a bad ground can cause multiple lighting issues and set this code.
  • Intermittent faults from loose connections or chafed wires may not set the code every time, so a wiggle test of the harness while monitoring live data can be more effective than static testing.
  • Some vehicles use a separate rear electronic module that controls the brake lights; a fault in that module can mimic a wiring issue and set B0851 without any problem in the bulb or wiring.

Can I keep driving with B0851?

It is generally safe to continue driving with B0851 because the right brake light is not critical for vehicle operation, but you should avoid relying on that light and get it repaired promptly to maintain full brake light visibility and avoid a traffic citation.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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