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DTC codes / B08xx · Airbags, seats, doors, lights and comfort systems / B0850

B0850Left Brake Lamp Circuit

B0850 is a body code indicating a fault in the left brake lamp circuit, which the body control module (BCM) monitors for expected voltage or current when the brake pedal is pressed. The computer detects an open circuit, short to ground, or short to battery in the wiring or bulb circuit, setting the code and often illuminating the brake system warning light.

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

Symptoms you may notice

  • The left brake light does not illuminate when the brake pedal is pressed.
  • The right brake light and center high-mount stop lamp may still work normally.
  • A red brake system warning light or a 'check brake lamps' message appears on the dashboard.
  • The cruise control may fail to engage or disengage unexpectedly if the brake light circuit is shared with the cruise control module.
  • Often no symptoms besides the warning light, especially if the faulty circuit is a secondary monitoring path.
  • The turn signal or hazard flasher on the left side may also be inoperative if the brake and turn signal share a bulb filament.

Common causes, most likely first

  1. Burned-out left brake light bulb or a bulb with a broken filament.
  2. Corroded, loose, or damaged left brake light socket or connector.
  3. Open circuit in the left brake lamp wiring, such as a broken wire or chafed harness.
  4. Short to ground or short to voltage in the left brake lamp circuit.
  5. Faulty brake pedal position sensor or brake light switch sending incorrect signals to the BCM.
  6. Internal failure in the body control module (BCM) affecting the left brake lamp output driver.
  7. Aftermarket LED bulb or incorrect bulb type causing a current draw mismatch that the BCM interprets as a fault.

How to diagnose and fix it

  1. Verify the left brake light operation with a helper pressing the pedal, and inspect the bulb for a broken filament or darkening.
  2. Replace the left brake light bulb with the correct OEM-type bulb, then clear the code and retest.
  3. Inspect the left brake light socket and connector for corrosion, melting, or loose pins; clean or repair as needed.
  4. Check the brake light fuse and the left brake lamp ground connection for continuity and tightness.
  5. Use a digital multimeter to measure voltage and resistance at the left brake lamp connector while the pedal is pressed and released, comparing to the right side.
  6. If wiring tests are normal, test the brake pedal position sensor signal and the BCM output for the left brake lamp circuit using a scan tool with bidirectional controls.
  7. Repair any damaged wiring or replace the BCM only after confirming all other components and circuits are functioning correctly.

What the BCM actually monitors on the left brake lamp

  • The BCM often monitors brake lamp circuits by sensing current flow or voltage drop, so a bulb that lights but draws the wrong current can still set B0850.
  • A faulty brake pedal position sensor can set B0850 even when the left brake light works, because the BCM compares switch inputs against expected lamp circuit feedback.
  • Aftermarket LED bulbs are a common cause of false B0850 codes because their low current draw falls outside the BCM's expected range, mimicking an open circuit.
  • The left brake lamp circuit often shares a bulb filament with the left turn signal on older vehicles, so a turn signal fault can be misdiagnosed as a brake lamp issue.
  • A shorted or damaged trailer wiring harness can back-feed the left brake lamp circuit and set B0850 without any visible problem at the rear lamps.

Can I keep driving with B0850?

You can usually drive the vehicle, but the left brake light may not work, reducing rear visibility to other drivers and potentially affecting cruise control operation. Avoid driving at night or in heavy traffic until the fault is repaired, and do not rely on the left brake light for signaling.

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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