B0850: Left Brake Lamp Circuit
A fault has been detected in the left brake lamp circuit.
What B0850 means
B0850 is a generic OBD-II body code in the lighting group. In plain English: the computer found an electrical fault in that circuit: an open, short or bad connection, or a failed component.
- Check engine light
- Usually off
- Emissions-related
- No
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
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Symptoms
- Warning light or symbol related to the affected system
- Comfort or safety function impaired
Common causes
- 01Burnt out brake lamp bulbMost likely
- 02Brake light switch faultPossible
Can I drive with B0850?
Generally yes.
This fault is unlikely to affect how the car drives. Fix it when convenient to restore the affected feature.
Why we rate it low
- Does not normally turn on the check engine light
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose B0850
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just B0850. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Fix other faults first
If other codes are stored, start with power-supply, sensor-circuit and communication codes. They can cause this code as a side effect.
- 3
Test the circuit
Inspect the wiring and connector at the left brake lamp and body control module for damage, corrosion or chafing. With a multimeter, check supply voltage, ground and signal against the specifications in the service manual for your vehicle.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Burnt out brake lamp bulb; brake light switch fault. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
B0850 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Easy
- Good DIY job?
- Yes, with a scan tool
Basic hand tools; a common first DIY job.
Parts and systems involved
- Left brake lamp
- Body control module
B0850 FAQ
What does B0850 mean?
B0850 means “Left Brake Lamp Circuit”. A fault has been detected in the left brake lamp circuit.
Is B0850 serious? Can I keep driving?
We rate B0850 as low severity. Generally yes. This fault is unlikely to affect how the car drives. Fix it when convenient to restore the affected feature.
What causes B0850?
The most common causes, from most to least likely: Burnt out brake lamp bulb; Brake light switch fault. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix B0850?
Typical labor is about $25–$125 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix B0850 myself?
DIY difficulty is easy. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will B0850 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.