Code
P0578
HUMMER
P — Powertrain
Cruise Control Related Malfunction
AI status
Completed
Completed
100%
Causes
- Faulty brake light / cruise disable switch (brake switch)
- Open, corroded or damaged wiring or connectors in the brake/cruise circuit
- Poor or missing ground at the BCM/PCM or switch return
- Aftermarket cruise or trailer wiring interfering with the circuit
- Blown fuse or poor power supply to related module
- Faulty body/engine control module or intermittent software issue
Symptoms
- Cruise control inoperative or disabled
- Cruise indicator lamp may blink or show a fault
- Brake lights may behave incorrectly (not lighting or stuck on)
- MIL may or may not be illuminated; code stored in memory
- Intermittent loss of cruise control or immediate disengagement when trying to set
What to check
- Read freeze frame and permanent data with a scan tool; note conditions when code set
- Visually inspect brake pedal switch, bracket, and connectors for damage, corrosion, or misadjustment
- Verify brake lights function when pedal is pressed
- Inspect wiring harness from brake switch to module for chafing, pinched wires, or aftermarket splices
- Check fuses and power/ground at the module and switch connector
- Backprobe switch and module connector while operating pedal and observe signals with a DMM or lab scope
Signal parameters
- Brake switch state vs. pedal: logical change when pedal pressed/released (open/closed)
- Typical voltages: 0 V (ground) and ~battery voltage (~12 V) depending on circuit pull-up/pull-down — verify with service data
- Continuity/resistance of switch when closed should be very low (near 0 Ω)
- Scan-tool live data: cruise enable / brake input status should toggle with pedal
Diagnostic algorithm
- Connect a scan tool, read DTC P0578 and related codes; record freeze-frame data
- Visually inspect brake switch mounting and connector; verify brake lights operate correctly
- With key ON (engine off) backprobe the brake switch pins and observe voltage/state while pressing/releasing the pedal; compare to expected changes
- Trace and inspect the wiring between the switch and the control module; repair any damaged wires or corroded terminals
- If wiring and switch are good, disconnect the switch and measure for expected pull-up/pull-down at the module harness to confirm module input behavior
- Repair or replace the brake switch and related connector if confirmed faulty; clear codes and road-test to verify the code does not return
- If fault persists after wiring and switch repairs, consider module diagnostics or reprogramming per manufacturer procedures (do not replace module without conclusive tests)
Likely causes
- Failed or misaligned brake light / brake pedal position switch
- Corroded connector or broken wire at the switch harness
- Short to battery or open circuit on the switch feed or return
- Aftermarket device (tow harness, alarm, remote start) tied into brake or cruise wiring
- Module input fault after confirming wiring integrity
Fault status
Status
Control module detected an abnormal high/open condition on the cruise/brake input circuit (P0578). Cruise may be disabled; inspection of brake switch and wiring is recommended.
Repair difficulty: Medium
Diagnostic time: 0.5 - 2.0 hours
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Repair manuals
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138
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