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DTC codes / GM codes / P1574

P1574 · GMStoplamp Switch Circuit

P1574 on GM vehicles indicates that the powertrain control module (PCM) has detected an implausible signal from the stoplamp switch circuit, which is used to inform the computer when the brake pedal is pressed. The PCM expects a specific voltage pattern when the brake pedal is released and pressed, and this code sets when the signal does not match the expected state, often due to a faulty switch, wiring issue, or misadjusted pedal.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • The check engine light or service vehicle soon light illuminates on the dashboard.
  • Often no symptoms besides the warning light, and the vehicle drives normally.
  • The brake lights may fail to illuminate when the brake pedal is pressed.
  • The brake lights may stay on continuously even when the pedal is released.
  • Cruise control may become inoperative or disengage unexpectedly.
  • The transmission may shift harshly or fail to engage overdrive due to incorrect brake input.
  • The engine may hesitate or stall when coming to a stop if the PCM misinterprets the brake signal.

Common causes, most likely first

  1. Faulty stoplamp switch (brake pedal position switch) that has worn contacts or internal failure.
  2. Misadjusted or loose stoplamp switch mounting, causing incorrect signal to the PCM.
  3. Damaged or corroded wiring or connectors in the stoplamp switch circuit, including the ground circuit.
  4. Blown fuse that supplies power to the stoplamp switch circuit.
  5. Brake pedal assembly wear or binding that prevents the switch from actuating properly.
  6. Aftermarket accessories (e.g., trailer wiring, alarm systems) improperly tapped into the brake light circuit.
  7. PCM software issue or internal fault, though this is less common and should be diagnosed last.

How to diagnose and fix it

  1. Check for any aftermarket wiring or accessories tapped into the brake light circuit and disconnect them temporarily to see if the code returns.
  2. Inspect the stoplamp switch fuse and replace if blown, then clear the code and test.
  3. Visually inspect the stoplamp switch and its connector for damage, corrosion, or loose pins; clean or repair as needed.
  4. Test the stoplamp switch operation with a multimeter or scan tool to verify it toggles correctly when the brake pedal is pressed and released.
  5. Adjust or replace the stoplamp switch according to GM service procedures, ensuring proper contact with the brake pedal arm.
  6. Check the wiring harness for continuity and shorts between the switch, PCM, and ground; repair any damaged wires.
  7. If all else fails, reprogram the PCM or replace it only after confirming the switch and wiring are functioning correctly.

Common Misdiagnosis Traps for P1574

  • A misadjusted stoplamp switch is a frequent cause that many technicians overlook; the switch may click but not make electrical contact at the correct pedal position.
  • Aftermarket LED brake light bulbs or trailer wiring can alter the current draw and cause the PCM to set P1574 even when the switch is good.
  • The stoplamp switch circuit often shares power with the cruise control and shift interlock systems, so a failure there can cause multiple symptoms beyond brake lights.
  • A weak or failing battery or charging system can cause voltage spikes that mimic a faulty stoplamp switch signal, so always check battery health first.
  • On some GM models, the stoplamp switch is part of a multifunction assembly that also includes the clutch pedal switch on manual transmission vehicles, leading to confusion.

Can I keep driving with P1574?

You can generally drive with P1574, but be aware that brake lights may not function correctly, which is a safety hazard. Avoid driving if brake lights are inoperative, and have the issue diagnosed promptly to prevent potential transmission or cruise control problems.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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