A school bus's yellow ABS malfunction lamp performs a bulb check and turns off at start-up, but then stays illuminated after that, or comes on while driving. What does this indicate, and what should the driver do?

AAMVA CDL Manual, Section 10.6.4 (Braking if ABS is Not Working), p. 10-9

Drivers need to know that an illuminated ABS lamp is a service reminder, not an emergency stop-the-bus situation, so they don't overreact or, worse, ignore it entirely. Understanding the bulb-check behavior also helps a driver tell a normal start-up flash from an actual malfunction.