Code
B1840
HUMMER
B — Body
Driver Lumbar Up Switch Circuit Malfunction
AI status
Completed
Completed
100%
Causes
- Faulty driver lumbar up switch
- Intermittent or open circuit in switch wiring or connector
- Corroded or pushed-out connector pins
- Short to battery or ground in the switch circuit
- Blown related fuse or poor fuse connection
- Faulty seat control module (seat ECU) or BCM input
Symptoms
- Driver lumbar up function inoperative or intermittent
- Possible other seat switch functions affected (dependant on vehicle wiring)
- DTC B1840 stored and lamp/notification may appear in body system diagnostics
- Seat adjustment may behave unexpectedly or not respond
What to check
- Retrieve and record all seat and body DTCs with a scan tool; note freeze frame/live data if present
- Visual inspection of driver seat, switch bezel and connector for damage or contamination
- Verify related fuses and relays in the interior fuse panel
- Operate lumbar up switch while watching live data for the seat module (if supported)
- Probe switch connector with backprobe or pinned adapter while operating switch to observe signal change
- Measure continuity between the switch terminals and the seat control module connector with connector(s) unplugged
Signal parameters
- Reference voltage typically present at switch connector: ~5 V (some systems may use key-on battery voltage — consult vehicle service manual)
- Switch closed (actuated) resistance: near 0–2 ohms between switch terminals (or switch pulls signal to ground)
- Switch open resistance: very high (>>1 MΩ) or open circuit
- Signal change when operating switch: voltage should toggle between reference voltage and ground (or a defined logic level)
- Current draw for switch circuit: very low (milliamps)
Diagnostic algorithm
- Connect a diagnostic scan tool; record B1840 and any related codes and available live data.
- Perform a soft reset: clear the code, operate lumbar up switch several times, and see if code returns to rule out transient condition.
- Visually inspect driver lumbar up switch, bezel and wiring for damage or contamination; inspect under-seat harness routing for chafing and broken wires.
- Check fuses related to seat power/controls; replace if blown and inspect for underlying cause.
- Backprobe the switch connector with ignition ON and operate the switch: verify reference voltage present and that the signal changes when switch is pressed.
- Disconnect switch and measure switch continuity with a multimeter while actuating: verify appropriate open/closed resistance.
- If reference voltage is present but switch does not change signal, replace the lumbar up switch and retest.
- If switch tests good, check continuity and resistance of the wiring between the switch connector and the seat control module connector; repair any open/shorts found.
- If wiring and switch are good but fault remains, verify proper ground(s) for the seat module and inspect module connector pins for corrosion/push-out.
- If all wiring and switches check good, suspect seat control module input fault — consult manufacturer procedures for module testing, reprogramming, or replacement.
- After repairs, clear codes and perform functional test of lumbar up/down and other seat functions; road test/repeat cycles to confirm repair.
Likely causes
- Failed lumbar up switch (most common)
- Broken/abraded wiring harness inside seat (due to seat movement)
- Poor connector pin contact or corrosion at switch connector
- Intermittent short to ground on the switch signal wire
Fault status
Status
Driver Lumbar Up Switch Circuit Malfunction — the seat lumbar 'up' switch circuit is reporting an electrical fault (open, short, or intermittent) to the seat control module.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
Similar codes
Repair manuals
Manual library for HUMMER
138
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Code
B1840
Other
B — Body
Wiper Front Power Circuit Failure
AI status
Completed
Completed
100%
Causes
- Faulty driver lumbar up switch
- Intermittent or open circuit in switch wiring or connector
- Corroded or pushed-out connector pins
- Short to battery or ground in the switch circuit
- Blown related fuse or poor fuse connection
- Faulty seat control module (seat ECU) or BCM input
Symptoms
- Driver lumbar up function inoperative or intermittent
- Possible other seat switch functions affected (dependant on vehicle wiring)
- DTC B1840 stored and lamp/notification may appear in body system diagnostics
- Seat adjustment may behave unexpectedly or not respond
What to check
- Retrieve and record all seat and body DTCs with a scan tool; note freeze frame/live data if present
- Visual inspection of driver seat, switch bezel and connector for damage or contamination
- Verify related fuses and relays in the interior fuse panel
- Operate lumbar up switch while watching live data for the seat module (if supported)
- Probe switch connector with backprobe or pinned adapter while operating switch to observe signal change
- Measure continuity between the switch terminals and the seat control module connector with connector(s) unplugged
Signal parameters
- Reference voltage typically present at switch connector: ~5 V (some systems may use key-on battery voltage — consult vehicle service manual)
- Switch closed (actuated) resistance: near 0–2 ohms between switch terminals (or switch pulls signal to ground)
- Switch open resistance: very high (>>1 MΩ) or open circuit
- Signal change when operating switch: voltage should toggle between reference voltage and ground (or a defined logic level)
- Current draw for switch circuit: very low (milliamps)
Diagnostic algorithm
- Connect a diagnostic scan tool; record B1840 and any related codes and available live data.
- Perform a soft reset: clear the code, operate lumbar up switch several times, and see if code returns to rule out transient condition.
- Visually inspect driver lumbar up switch, bezel and wiring for damage or contamination; inspect under-seat harness routing for chafing and broken wires.
- Check fuses related to seat power/controls; replace if blown and inspect for underlying cause.
- Backprobe the switch connector with ignition ON and operate the switch: verify reference voltage present and that the signal changes when switch is pressed.
- Disconnect switch and measure switch continuity with a multimeter while actuating: verify appropriate open/closed resistance.
- If reference voltage is present but switch does not change signal, replace the lumbar up switch and retest.
- If switch tests good, check continuity and resistance of the wiring between the switch connector and the seat control module connector; repair any open/shorts found.
- If wiring and switch are good but fault remains, verify proper ground(s) for the seat module and inspect module connector pins for corrosion/push-out.
- If all wiring and switches check good, suspect seat control module input fault — consult manufacturer procedures for module testing, reprogramming, or replacement.
- After repairs, clear codes and perform functional test of lumbar up/down and other seat functions; road test/repeat cycles to confirm repair.
Likely causes
- Failed lumbar up switch (most common)
- Broken/abraded wiring harness inside seat (due to seat movement)
- Poor connector pin contact or corrosion at switch connector
- Intermittent short to ground on the switch signal wire
Fault status
Status
Driver Lumbar Up Switch Circuit Malfunction — the seat lumbar 'up' switch circuit is reporting an electrical fault (open, short, or intermittent) to the seat control module.
Repair difficulty: Medium
Diagnostic time: 0.5-1.5 hours
Similar codes
Repair manuals
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