What it means
The brake light switch signal seen by the ECU is implausible — for example, both contacts active simultaneously, or no signal change despite the brake pedal being pressed (cruise control engages and disengages on this signal). On many cars also disables shift interlock.
At a glance
- Usually lights the check-engine light
How it works
Brake-switch circuit A carries a brake-pedal state used by the control module to disengage cruise control; the signal may also be used by the shift interlock. The module sets this code when the A signal is implausible, such as when it fails to change as the pedal is pressed or conflicts with another brake-switch state.
Common causes
Most likely first.
- Brake switch failed (worn contacts, internal break)
- Switch misadjusted (does not actuate at correct pedal position)
Symptoms
- Check-engine light may come on
- Cruise control may not engage or may disengage unexpectedly
- Brake lights may stay on or fail to illuminate
- Shift interlock may not release on vehicles that use this signal
How to diagnose it
- With the vehicle parked, check whether all brake lights illuminate when the pedal is pressed and turn off when it is released.
- Use a scan tool to watch brake-switch A and any related brake-switch states while slowly pressing and releasing the pedal; look for missing or conflicting changes.
- Inspect the brake switch at the pedal for looseness, damage, or misadjustment that prevents it from actuating at the correct pedal position.
- Check the switch connector and wiring for loose terminals, corrosion, chafing, or an open or short circuit.
- Test the switch contacts and circuit against the manufacturer's specification, then check the shift-interlock signal if that feature is affected.
Common fixes
- Adjust a mispositioned brake switch so it changes state correctly with pedal movement.
- Reconnect or clean a loose or corroded brake-switch connector.
- Repair damaged or open wiring in the brake-switch A circuit.
- Replace a brake switch with worn or internally failed contacts.
- Repair the related control-module circuit if the switch and wiring test correctly.
Can I keep driving?
Check the brake lights before driving; if they do not work, stay on, or the brake pedal feels abnormal, do not drive until the fault is corrected. Cruise may disengage unpredictably, and the shift interlock may also be affected.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.