B16B7 – Center high-mounted brake light output malfunction (Mercedes-Benz)

B16B7 means your Mercedes-Benz has detected a problem with the center high-mounted brake light output. You may lose the third brake light, or it may work intermittently. That reduces rear visibility and can create a safety issue. According to Mercedes-Benz factory diagnostic data, this code indicates a center high-mounted brake light output malfunction. In plain terms, a control module tried to command the light on and did not see the expected electrical result. This code does not prove the LED unit is bad. It points you toward the output circuit so you can test power, ground, load, and wiring first.

B16B7 Quick Answer

B16B7 on a Mercedes-Benz Sprinter 907 indicates a malfunction in the center high-mounted brake light output circuit. Confirm the brake light command and the circuit’s ability to carry current before replacing the lamp or a module.

What Does B16B7 Mean?

Official definition: Center high-mounted brake light output malfunction. In practice, the vehicle detected that the third brake light output did not behave correctly when the brakes were applied or when the module ran an output check.

What the module actually checks: the control unit monitors the electrical behavior of the brake light output it drives, such as load presence, excessive current draw, or an abnormal voltage state. Why that matters: the same code can result from an open circuit, high resistance, a short to ground, a short to battery, water intrusion at the rear door area, or a poor ground. The DTC identifies the suspected trouble area, not the root cause.

Theory of Operation

Under normal operation, the brake pedal input and brake request logic trigger a body-controlled output for the center high-mounted brake light. The module then supplies voltage or ground to the lamp circuit, depending on Mercedes-Benz design for that platform.

B16B7 sets when the module commands the output and sees an unexpected electrical response. An open circuit can prevent current flow. A short or water intrusion can overload the driver. High resistance in the rear harness can also fool load monitoring and create an intermittent fault.

Symptoms

Drivers and technicians usually notice one or more of these brake light related symptoms:

• Inoperative CHMSL the center high-mounted brake light does not illuminate when braking
• Intermittent operation the third brake light works sometimes, then fails over bumps or door movement
• Bulb-out message the cluster or message center reports an exterior light malfunction
• Rapid re-fail the light works after clearing codes, then fails again during the next brake event
• Water-related failure the problem appears after washing, rain, or high humidity
• Odd glow or dim light the CHMSL glows faintly, flickers, or looks dim compared to normal
• Multiple rear light issues other rear lamps show intermittent faults if they share a harness section or ground

Common Causes

• Open circuit to the CHMSL lamp: A break in the output wire stops current flow, so the module sees no load or no expected feedback.
• Short to ground in the CHMSL output: Chafed insulation or a pinched harness pulls the output low and can trigger overcurrent protection in the driver.
• Short to battery voltage on the output: Backfeeding from another lamp circuit or wiring damage holds the line high when the module commands it off.
• High resistance at the CHMSL connector: Corrosion or a spread terminal limits current, so the lamp looks dim or fails while the module flags an output malfunction.
• Center brake lamp assembly fault: An internal open, water intrusion, or LED driver failure changes the load characteristics and trips the module’s plausibility check.
• Water intrusion in rear door or roof harness area: Moisture migrates into splices and connectors, creating intermittent opens and shorts during vibration or temperature changes.
• Incorrect bulb/LED unit or aftermarket wiring: Non-OE lamp designs or add-on trailer/aux wiring can alter current draw and confuse the Mercedes-Benz output monitoring logic.
• Fuse or power distribution issue feeding the lamp driver: A weak fuse connection or supply fault can let the system work briefly and then drop out under load.
• Control module output driver fault: A failed high-side/low-side transistor in the controlling module can prevent proper switching even with good wiring and a good lamp.

Diagnosis Steps

Use a scan tool that can read Mercedes-Benz body faults and run output tests. Have a quality DVOM, a test light rated for automotive use, and back-probing leads. Keep wiring diagrams and connector pinouts on hand for the Sprinter 907 platform you service. Plan to do voltage-drop tests under load, not continuity checks alone.

1. Confirm B16B7 and record DTC status and history. Save freeze frame data if the module provides it, and note battery voltage, ignition state, and whether the brake pedal input showed applied. Freeze frame shows conditions when the fault set, while a scan-tool snapshot you trigger later helps catch an intermittent during a road test.
2. Perform a fast visual inspection before meter work. Check the CHMSL lens for water, cracks, and loose mounting. Inspect the harness routing at the rear door hinges or roof pass-through areas for rub-through, pinched sections, and recent body work.
3. Check fuses and power distribution that feed the brake lamp/CHMSL output circuit. Verify the fuse element and also verify tight terminal tension at the fuse and any related power distribution points. A fuse can test “good” yet drop voltage under load from heat or poor contact.
4. Verify the control module power and grounds under load using voltage-drop testing. Command brake lamps on and measure ground drop at the module ground path. Keep the circuit operating and confirm less than 0.1V drop on the ground side; high resistance can mimic an output failure.
5. Use the scan tool to run an actuator test or output test for the center high-mounted brake light. Watch for immediate re-setting of B16B7 during the command. A hard fault often returns right away during key-on or output test on continuously monitored body outputs.
6. Check the brake pedal input status on the scan tool while pressing and releasing the pedal. Confirm the module sees a clean transition and stable “applied” state. A missing brake request can mislead diagnosis, even though this DTC targets the CHMSL output circuit.
7. At the CHMSL connector, back-probe the power and ground (or command and return, depending on design) while commanding the lamp on. Look for a solid supply and a solid ground under load, not just open-circuit voltage. If voltage appears but the lamp stays dark, load the circuit with a test light and watch for voltage collapse.
8. Perform a voltage-drop test across the CHMSL connector and any accessible splices while the lamp should be on. Measure from the module side of the feed to the lamp side of the feed, then do the same on the ground path if the lamp uses a dedicated ground. A significant drop points to corrosion, a loose terminal, or a damaged splice.
9. If the circuit shows low or unstable voltage at the lamp, isolate the fault by checking closer to the module output pin. Compare readings at the module connector versus the lamp connector to locate the section with the loss. Do not replace the lamp or module until you prove where the voltage drop occurs.
10. If the circuit shows correct voltage and ground at the CHMSL but the lamp does not light, verify lamp integrity. Substitute a known-good lamp unit if practical, or bench-test the lamp per service information. If the lamp assembly contains LEDs, avoid assumptions based on continuity; verify operation under proper polarity and load.
11. Clear DTCs and run the same actuator test again. Confirm the CHMSL operates repeatedly and that B16B7 does not return as pending or stored. If the fault was intermittent, use a scan-tool snapshot during a road test and lightly manipulate the suspect harness area to confirm stability.

Professional tip: Do not trust continuity checks on Sprinter rear lighting faults. Corroded terminals often pass continuity with milliamps, then fail under lamp load. Use voltage-drop testing with the circuit commanded on, and load the circuit with a test light when you suspect a high-resistance connection.

Possible Fixes

• Repair damaged harness routing: Repair chafed, pinched, or broken wiring near hinges, roof pass-throughs, or rear door harness guides, then secure it to prevent repeat rub-through.
• Restore connector integrity: Clean corrosion, repair water intrusion sources, and replace terminals with poor tension at the CHMSL connector or intermediate connectors.
• Repair power distribution faults: Correct loose fuse fitment, overheated fuse terminals, or failing power feed points that drop voltage under load.
• Replace the CHMSL assembly only after circuit proof: Install a correct-spec Mercedes-Benz lamp unit once you verify proper feed and ground but the lamp fails to operate.
• Remove aftermarket backfeed sources: Correct trailer/aux lighting taps, incorrect splices, or add-on modules that backfeed voltage into the CHMSL output line.
• Control module driver repair only after verification: If the output pin never switches and the circuit tests good end-to-end, follow Mercedes-Benz test plans for module output confirmation before replacement or programming.

Can I Still Drive With B16B7?

You can usually drive a Mercedes-Benz Sprinter 907 with B16B7, but you should treat it as a safety-related lighting fault. This code points to a malfunction in the center high-mounted brake light output circuit. That lamp warns drivers behind you during braking. If it does not light, rear-end risk increases, especially in rain, glare, or heavy traffic. Before any long trip, confirm brake lamp operation with a helper or by backing near a reflective surface. If the center lamp stays off, drive only as needed to reach a repair location. Also verify the left and right brake lamps work, since some faults affect more than one output depending on the platform’s body control strategy.

How Serious Is This Code?

B16B7 ranges from an inconvenience to a true safety concern. It feels minor if only the center brake lamp fails and the other brake lamps work. It becomes serious when the vehicle shows intermittent brake lamp operation, bulb-out warnings, or multiple rear lighting issues. Mercedes-Benz body electronics can disable or limit an output when it sees a short, overload, or implausible current draw. That protective action can change the symptom as conditions shift. This is not a drivability code, so the engine and transmission typically operate normally. Still, treat any brake light fault as high priority. A correct fix requires circuit confirmation first, not guessing a lamp assembly.

Common Misdiagnoses

Technicians often replace the center high-mounted lamp assembly first because it is accessible and the description sounds definitive. That wastes time when the real issue sits in the harness at the rear door hinge area, a partially backed-out connector pin, or water intrusion at the lamp connector. Another common miss involves ignoring the control-side diagnosis. The body control module output can shut down after detecting an overcurrent event, then recover after a key cycle. If you only test with the key cycled, you can miss an active short. Avoid these errors by load-testing the circuit, checking voltage drop at the ground under load, and comparing commanded output to actual current draw in scan-tool data when available.

Most Likely Fix

The most common confirmed repair directions for B16B7 involve restoring correct circuit integrity to the center high-mounted brake light. Start with a tight inspection of the lamp connector, the rear door or body harness flex points, and any aftermarket wiring splices. Many Sprinter lighting faults come from high resistance, corrosion, or broken conductors that only fail with door movement. If testing proves the wiring and lamp load are correct, then verify the body controller’s output can supply power under load without shutting down. Only after those checks should you consider a module output driver fault, which may require programming and variant coding after replacement on Mercedes-Benz platforms.

Repair Costs

Repair cost depends on whether the root cause is a wheel speed sensor, wiring, connector condition, or the hydraulic control unit. Start with electrical checks before replacing brake system components.

Last updated: March 30, 2026