Code
C1291
HUMMER
C — Chassis
Open Brake Lamp Switch Contacts During Deceleration
Views:
UK: 14
EN: 16
RU: 28
AI status
Completed
Completed
100%
Causes
- Faulty or misadjusted brake lamp (stop) switch
- Broken, corroded or disconnected wiring/connector for the brake switch
- Poor ground or power feed to the switch or control module
- Intermittent contact in the switch plunger (mechanical wear or contamination)
- Aftermarket trailer wiring or rear-end lighting alterations interfering with the circuit
- Faulty Body Control Module (BCM) or ABS/ESC module input circuitry
Symptoms
- ABS/ESC or Traction Control Warning lamp illuminated
- Brake lights may not illuminate consistently during deceleration
- Cruise control may disengage or not allow set
- Loss of certain traction/ABS-related functions or degraded stability control
- Diagnostic trouble code stored (C1291) and related codes possible
What to check
- Scan vehicle and record freeze frame/data stream while code present; monitor 'brake switch' or 'stop lamp' status during deceleration
- Visually inspect brake lights operation while someone observes at rear during pedal application and deceleration
- Inspect brake switch plunger and mounting for proper adjustment and full travel
- Inspect connector and wiring for corrosion, pin damage, chafing, or broken wires
- Check fuse(s) and power/ground at switch and relevant control modules
- Wiggle test wiring and connectors while monitoring switch signal to reproduce intermittent opening
Signal parameters
- Brake switch signal should change state reliably when pedal is applied and released; ECU/ABS expects a clean transition
- Typical expected voltage levels: near battery voltage (~12 V) for one switch state and near 0 V for the other (may be inverted or referenced to module input — verify with vehicle wiring diagram)
- No rapid oscillation or loss of signal during deceleration events
- Continuity between switch and module terminals when circuit closed; open (high resistance) when circuit open
Diagnostic algorithm
- Connect scan tool and note the brake switch/stop lamp status in live data. Recreate the condition by safe low-speed deceleration while monitoring the data to confirm an open during decel.
- Verify brake lamps operate every time the brake pedal is applied. If lamps are intermittent, inspect bulbs, sockets, and rear harness.
- Visually inspect the brake light switch for correct installation and plunger travel. Adjust or actuate by hand and observe the live data for proper state change.
- With ignition off, visually inspect and disconnect the switch connector. Check pins for corrosion, bent terminals, or damage. Reconnect and retest.
- Measure voltage and continuity at the switch connector: verify power feed, ground (if applicable), and signal continuity to the BCM/ABS connector. Repair any open/corroded wiring.
- If wiring and switch test good, bench-test or replace the brake light switch and clear codes. Road test and monitor for recurrence.
- If fault persists after new switch and wiring verified, inspect BCM/ABS module input and ground circuits. Check for related communication codes (U-codes) and consider module testing or replacement per factory procedures.
- After repair, clear codes and perform a test drive including deceleration scenarios to confirm code does not return.
Likely causes
- Brake lamp switch misadjusted or failed (most common)
- Wiring/connector corrosion or broken conductor between switch and BCM/ABS
- Intermittent connector pin contact caused by movement during deceleration
- Aftermarket trailer or rear harness modification backfeeding or opening the circuit
- Less likely: internal module input fault
Fault status
Status
ABS/ESC module detected an open circuit on the brake lamp switch input during deceleration; stability control functions may be limited and brake light operation may be affected.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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