ABS indicator light due to failed module or what?

I have an ABS indicator light: How common is this due to a failed ABS module? Is it more often a failed sensor or something else?

2005 3500 dually with a healthy portion of rust underneath, especially on the brake lines.

Thanks.

It is not necessarily a module. They are generally reliable.

Without more information all answers will be guesses or lists of common things others have have found.

Begin by getting a scan for DTC's using a Sprinter specific scan tool.

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