Powertrain, generic · Transmission

P0724 Brake Switch B Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

Brake switch B is the brake signal to the transmission module or a second brake switch channel, used to release the converter clutch, adapt shift strategy on grades and cancel cruise control. Low and high describe the signal voltage, and what that means depends on polarity: GM's converter clutch brake switch is closed at rest (12 V), while Saab's Trionic 8 sees 12 V when braking. Detection is always a plausibility check against driving: the brake reads applied during accelerations or never applied during decelerations. P0724 means the signal stays high. For GM that reads as brake never applied: 2006 transmission modules flag it after eight decelerations without a brake signal of at least 2 s, without MIL. Saab's Trionic 8 flags a brake light signal that reads on while accelerating from 3 to 40 km/h, four times, and disables cruise control. The opposite is P0719.

Likely causes

Brake light switch sticking or misadjusted
Signal wire shorted to positive (Saab T8) or switch permanently closed (GM)
Retrofitted trailer or LED lights feeding back
Module input faulty

Symptoms

  • Cruise control cannot be activated or switches off
  • Unusual shifting, converter clutch behaviour changed
  • No downshift on uphill grades (Subaru)
  • Brake lights stay off or on

Repair estimate

DifficultyEasyDIY possible
Parts and labour€20–150
Time0.3–1 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake switchbrake switch wiringtransmission control module

Related codes

P0703P0719P0571P0572P0573P0504

References: gsi.ext.gm.com, gsi.ext.gm.com, saabwisonline.com, saabwisonline.com.