Code
B2540
HUMMER
B — Body
Rear Foglamps Control Circuit
AI status
Completed
Completed
100%
Causes
- Blown fuse for rear fog lamps
- Burned-out rear fog lamp bulb
- Faulty fog lamp relay
- Open or shorted wiring in harness to rear fog lamp
- Corroded or loose connector at lamp or chassis ground
- Poor body or lamp ground
Symptoms
- Rear fog lamp does not illuminate when switched on
- Rear fog lamp works intermittently or only when position changes (e.g., door open/closed)
- Instrument cluster indicator for fog lamps may not illuminate or may flash
- Related fuses blow repeatedly
- Other exterior lighting anomalies if BCM output is affected
- DTC B2540 (and possibly related body codes) stored in BCM
What to check
- Retrieve stored DTCs and freeze-frame data with a diagnostic scanner; note any related body/network codes
- Visual inspection of rear fog lamp bulb and socket for corrosion, melted plastic, or water
- Check fuse(s) and relay for the rear fog lamp circuit; replace if blown and retest
- Backprobe lamp connector with switch ON to verify presence of battery voltage and ground
- Verify chassis/body ground at lamp mounting point and main ground straps for continuity and cleanliness
- Inspect wiring harness for chafing, pinched sections, or damage near bumper, tailgate hinge, or liftgate
Signal parameters
- Expected lamp connector voltage with lamp ON: approximately battery voltage (11–14 V)
- Expected lamp filament resistance (typical single filament rear fog bulb): low ohms (approx. 2–6 Ω) — consult OEM spec for exact value
- Control signal type from BCM: either switched battery (12 V) or ground-switched; some vehicles may use a low-voltage control or PWM from the BCM
- Expected current draw when ON: roughly 1–6 A depending on bulb wattage (measure with ammeter for exact value)
- Switch/command logic: fog lamp switch closed should result in a change at BCM input and a corresponding command output; scan tool should show switch state and lamp command
- CAN or module status: BCM should be communicating on the vehicle bus; loss of communication may present as additional network codes (e.g., U0140)
Diagnostic algorithm
- Connect a diagnostic scanner, read and record DTCs and freeze-frame data; note related network codes.
- Confirm symptom: turn fog lamp ON while observing dash indicator and lamp. Note whether lamp illuminates, flickers, or is dead.
- Inspect lamp bulb and socket. Replace bulb if visibly burned or if filament open. Retest.
- Check related fuse(s) and relay: inspect visually and test for continuity. Replace suspicious components and retest.
- With lamp switch ON, backprobe the lamp connector: verify presence of battery voltage at the power pin and a good ground at the ground pin. If voltage present and lamp fails, suspect bulb or poor contact.
- If no voltage at connector, trace wiring back toward relay/BCM. Check continuity to relay and fuse using an ohmmeter. Repair any opens or damaged insulation.
- If voltage present only intermittently or under certain conditions, perform wiggle test on harness/connector and inspect for corrosion or loose terminals. Clean/repair as needed.
- Command the fog lamp ON via scan tool/function test to determine if BCM is issuing the output. If scan tool shows a command but no voltage at lamp, suspect wiring/relay/connector between BCM and lamp.
- Check grounds: measure voltage drop between lamp ground and battery negative while lamp is ON; high drop indicates poor ground—clean and tighten ground connections.
- If wiring, connectors, fuse and relay check good but lamp still inoperative and BCM does not respond correctly, investigate BCM power/ground/communication circuits. Repair or replace BCM only after confirming external wiring and components are good.
- After repair, clear codes, operate the fog lamp through several cycles and re-scan to ensure code does not return. Road-test as needed to confirm intermittent issues fixed.
- Safety note: isolate battery power when repairing wiring or connectors; use appropriate PPE and follow shop electrical safety procedures.
Likely causes
- Burned-out bulb or blown fuse
- Corroded/loose connector or poor ground at rear lamp
- Open/shorted wiring in lamp harness near bumper or tailgate
- Faulty relay or control output from BCM
- Intermittent BCM or communication fault (less common)
Fault status
Status
Rear fog lamp control circuit malfunction detected. Possible causes include open/short in wiring, blown fuse, faulty bulb, bad ground, relay failure, or BCM/control module fault. Inspect fuse, bulb, connector, wiring and module communication to isolate the fault.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
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Code
B2540
Other
B — Body
Aux A/C Mode Position Reference Circuit Short to Battery
AI status
Completed
Completed
100%
Causes
- Blown fuse for rear fog lamps
- Burned-out rear fog lamp bulb
- Faulty fog lamp relay
- Open or shorted wiring in harness to rear fog lamp
- Corroded or loose connector at lamp or chassis ground
- Poor body or lamp ground
Symptoms
- Rear fog lamp does not illuminate when switched on
- Rear fog lamp works intermittently or only when position changes (e.g., door open/closed)
- Instrument cluster indicator for fog lamps may not illuminate or may flash
- Related fuses blow repeatedly
- Other exterior lighting anomalies if BCM output is affected
- DTC B2540 (and possibly related body codes) stored in BCM
What to check
- Retrieve stored DTCs and freeze-frame data with a diagnostic scanner; note any related body/network codes
- Visual inspection of rear fog lamp bulb and socket for corrosion, melted plastic, or water
- Check fuse(s) and relay for the rear fog lamp circuit; replace if blown and retest
- Backprobe lamp connector with switch ON to verify presence of battery voltage and ground
- Verify chassis/body ground at lamp mounting point and main ground straps for continuity and cleanliness
- Inspect wiring harness for chafing, pinched sections, or damage near bumper, tailgate hinge, or liftgate
Signal parameters
- Expected lamp connector voltage with lamp ON: approximately battery voltage (11–14 V)
- Expected lamp filament resistance (typical single filament rear fog bulb): low ohms (approx. 2–6 Ω) — consult OEM spec for exact value
- Control signal type from BCM: either switched battery (12 V) or ground-switched; some vehicles may use a low-voltage control or PWM from the BCM
- Expected current draw when ON: roughly 1–6 A depending on bulb wattage (measure with ammeter for exact value)
- Switch/command logic: fog lamp switch closed should result in a change at BCM input and a corresponding command output; scan tool should show switch state and lamp command
- CAN or module status: BCM should be communicating on the vehicle bus; loss of communication may present as additional network codes (e.g., U0140)
Diagnostic algorithm
- Connect a diagnostic scanner, read and record DTCs and freeze-frame data; note related network codes.
- Confirm symptom: turn fog lamp ON while observing dash indicator and lamp. Note whether lamp illuminates, flickers, or is dead.
- Inspect lamp bulb and socket. Replace bulb if visibly burned or if filament open. Retest.
- Check related fuse(s) and relay: inspect visually and test for continuity. Replace suspicious components and retest.
- With lamp switch ON, backprobe the lamp connector: verify presence of battery voltage at the power pin and a good ground at the ground pin. If voltage present and lamp fails, suspect bulb or poor contact.
- If no voltage at connector, trace wiring back toward relay/BCM. Check continuity to relay and fuse using an ohmmeter. Repair any opens or damaged insulation.
- If voltage present only intermittently or under certain conditions, perform wiggle test on harness/connector and inspect for corrosion or loose terminals. Clean/repair as needed.
- Command the fog lamp ON via scan tool/function test to determine if BCM is issuing the output. If scan tool shows a command but no voltage at lamp, suspect wiring/relay/connector between BCM and lamp.
- Check grounds: measure voltage drop between lamp ground and battery negative while lamp is ON; high drop indicates poor ground—clean and tighten ground connections.
- If wiring, connectors, fuse and relay check good but lamp still inoperative and BCM does not respond correctly, investigate BCM power/ground/communication circuits. Repair or replace BCM only after confirming external wiring and components are good.
- After repair, clear codes, operate the fog lamp through several cycles and re-scan to ensure code does not return. Road-test as needed to confirm intermittent issues fixed.
- Safety note: isolate battery power when repairing wiring or connectors; use appropriate PPE and follow shop electrical safety procedures.
Likely causes
- Burned-out bulb or blown fuse
- Corroded/loose connector or poor ground at rear lamp
- Open/shorted wiring in lamp harness near bumper or tailgate
- Faulty relay or control output from BCM
- Intermittent BCM or communication fault (less common)
Fault status
Status
Rear fog lamp control circuit malfunction detected. Possible causes include open/short in wiring, blown fuse, faulty bulb, bad ground, relay failure, or BCM/control module fault. Inspect fuse, bulb, connector, wiring and module communication to isolate the fault.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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Repair manuals
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