Why is the ABS light on in a sedan?

[Sienna]

When the ABS light is displayed on a car, it indicates a malfunction in the vehicle's anti-lock braking system. If the wheels lock up during emergency braking, the vehicle may lose control and skid, making it impossible for the driver to decelerate or adjust the vehicle's trajectory, posing significant safety hazards. There are several states in which the ABS light may illuminate, each with different causes and solutions: State 1: ABS warning light stays on continuously (most common). Cause 1: The ABS wheel speed sensor's sensing area is covered by dirt, mud, or other contaminants, affecting the sensor's ability to detect the corresponding speed signal. This prevents the ABS computer from determining the vehicle speed and the wheel slip rate, thus failing to issue appropriate commands to regulate braking. Solution: Clean the dirt from the wheel speed sensor and adjust the gap between the sensor and the signal ring to restore normal function. Cause 2: Loose connections in the system wiring or poor contact in the ABS relay lead to signal issues, causing system failure. Solution: Check the wiring connections and reconnect any loose parts. State 2: ABS warning light intermittently illuminates and turns off when accelerating. Cause: When multiple vehicle electrical components are in use, the battery voltage drops below 10.5V. As the engine speed increases, the voltage rises, and the ABS light turns off. This may also occur if the ABS system's power supply voltage is too low due to poor terminal contact or grounding issues. Solution: Check the battery's specific gravity, inspect the charging system, and examine the power supply (e.g., voltage relay or poor power contact). State 3: ABS warning light stays on after engine start and only turns off when the engine is turned off (IGFF). Cause: Poor grounding contact in the ABS hydraulic valve body, faulty wiring connections in the ABS hydraulic valve body, or ABS computer malfunction. Solution: Loosen and retighten the grounding screw on the hydraulic valve body, cleaning the contact surface if necessary. Check for enlarged gaps in the connectors and replace the ABS or ABS/ASR computer if needed. State 4: ABS warning light illuminates during high-speed driving. Cause: At high speeds, the ABS computer detects a significant discrepancy between the rear and front wheel speed signals, or incorrect tire or wheel rim specifications. Solution: Refer to the vehicle's tire and wheel rim specifications, typically found on the label near the fuel tank cap. State 5: ABS warning light intermittently illuminates. Cause: Improper adjustment of the brake light switch or poor internal contact in the brake light. Solution: With the engine running, press the brake pedal, push the brake switch linkage down fully with your fingers, then release the brake pedal to complete the self-setting position. Replace the brake light switch if necessary.