What Does B2520 Mean?
This code indicates the Body Control Module (BCM) has detected a short to battery voltage in the high mount stop lamp (center brake light/CHMSL) circuit. The BCM detected voltage present on the lamp circuit when it should be off, indicating current is flowing from an unintended battery voltage source. This typically means the positive power wire for the center brake light has contact with constant 12V power somewhere in the circuit.
Common Causes
45%
Damaged or chafed wiring harness causing short to power wire, often at trunk/liftgate hinge area or near sharp metal edges
30%
Failed high mount stop lamp assembly with internal short to power
15%
Corroded or damaged connector terminals at lamp assembly allowing cross-circuit contact
10%
Aftermarket accessories or trailer wiring improperly tapped into brake light circuit
Diagnostic Steps
1
Step 1: Visually inspect the high mount stop lamp assembly and connector for water intrusion, corrosion, melted terminals, or physical damage. Disconnect the lamp connector and check for pushed-back pins or crossed terminals.
2
Step 2: With lamp disconnected and ignition off, use a digital multimeter to measure voltage at the BCM-side connector terminal for the CHMSL circuit. Reading of 12V confirms short to battery upstream of connector; 0V indicates fault is in lamp assembly.
3
Step 3: If voltage present at BCM-side connector, trace the wiring harness from lamp connector through trunk/liftgate hinge area to BCM, inspecting for pinched, chafed, or damaged wires. Pay special attention to flex points and areas where harness contacts metal.
4
Step 4: If no external damage found, check for aftermarket wiring splices or trailer harness connections. Disconnect any non-OEM wiring and retest circuit with multimeter for battery voltage.
5
Step 5: If fault isolated to lamp assembly, ohm-test the lamp unit between power terminal and ground with lamp removed. Infinite resistance is normal; low resistance confirms internal short.
6
Step 6: After repair, clear codes, cycle brake pedal multiple times, and verify CHMSL operates correctly without code returning. Monitor freeze frame data to confirm BCM no longer detects voltage when lamp should be off.
Estimated Repair Cost
$75 - $400
Parts + labor, varies by vehicle and location
This code indicates the Body Control Module (BCM) has detected a short to battery voltage in the high mount stop lamp (center brake light/CHMSL) circuit. The BCM detected voltage present on the lamp circuit when it should be off, indicating current is flowing from an unintended battery voltage sourc...
The most common cause of B2520 (High Mount Stop Lamp Circuit Short to Battery) is: Damaged or chafed wiring harness causing short to power wire, often at trunk/liftgate hinge area or near sharp metal edges
Typical repair costs for B2520 range from $75 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle is safe to drive but the center brake light may illuminate constantly or not function properly, reducing visibility of braking to following traffic. Repair should be completed within a few days to maintain proper brake light safety signaling.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to B2520 to identify the root cause.
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Quick Info
Category
Body
System
Exterior Lighting System (Body Control)
Difficulty
Type
Generic (SAE)
Recommended Tools
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