What is a likely indication of a failed ABS hydraulic modulator unit?

A likely indication of a failed ABS hydraulic modulator unit is the continuous illumination of the ABS warning light on the instrument panel, often accompanied by a diagnostic trouble code (DTC) related to the modulator. The ABS hydraulic modulator unit contains valves and a pump that control brake pressure to each wheel during ABS activation. If the unit fails, it can prevent the ABS from functioning properly. This can manifest as a complete loss of ABS functionality, where the wheels may lock up during hard braking, or as erratic ABS activation, where the system engages unnecessarily. Other symptoms might include unusual brake pedal feel, such as a hard or spongy pedal, and difficulty bleeding the brakes. Diagnostic trouble codes (DTCs) specific to the modulator unit, which can be read using a scan tool, will often be present, providing further confirmation of the failure. The vehicle's braking system may still function in a conventional manner (without ABS), but the loss of ABS functionality represents a significant safety concern, especially in slippery conditions.