What are the reasons for the ABS warning light staying on?

[McOscar]

ABS warning light can illuminate in various states, each with different causes and solutions as follows: State 1: ABS warning light stays on continuously (most common). Cause 1: The ABS wheel speed sensor's detection area is covered by mud, sludge, or other contaminants, affecting its ability to detect the corresponding wheel speed signal. This prevents the ABS control unit from determining the vehicle speed and 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 tone ring to restore normal function. Cause 2: Poor signal due to loose connections in the system wiring or poor contact in the ABS relay, leading to system malfunction. Solution: Check all wiring connections and reconnect any loose parts. State 2: ABS warning light illuminates intermittently, turning off when accelerating. Cause: When multiple electrical devices are in use, the battery voltage may drop below 10.5V. As engine speed increases and voltage rises, the ABS warning light turns off. This can 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, only turning off when the engine is turned off. Cause: Poor grounding contact in the ABS hydraulic valve body; poor electrical connector contact in the ABS hydraulic valve body; ABS control unit 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. Replace the ABS or ABS/ASR control unit if needed. State 4: ABS warning light illuminates during high-speed driving. Cause: The ABS control unit detects a significant difference between rear and front wheel speeds during high-speed driving. This can also occur due to incorrect tire or wheel rim specifications. Solution: Refer to the vehicle's specified tire and wheel rim sizes, as indicated on the label near the fuel filler cap. State 5: ABS warning light illuminates intermittently. Cause: Improper adjustment of the brake light switch; poor internal contact in the brake light. Solution: With the engine running, press the brake pedal and use a finger to push the brake switch linkage fully downward, then release the brake pedal to complete the self-setting process. Replace the brake light switch if necessary. If the ABS warning light is on, it is advisable not to continue driving. An illuminated ABS light indicates a malfunction in the vehicle's anti-lock braking system. During emergency braking, if the wheels lock up, the vehicle may skid uncontrollably, preventing the driver from slowing down or adjusting the vehicle's trajectory, posing significant safety risks.