ABS dash light diagnosis...
 

So, today I replaced the rear rotors, pads and brake wear sensors on my 2000 Boxster 5-speed. I also washed out the engine compartment per some posts in this forum. No issues there. After completing all the brake work, bleeding brake lines, etc. I started the car. ABS light comes on. Trying to figure out what the problem is. Didn't touch any part of the ABS sensors during the brake work. Have read this could be as simple as a battery problem. Have already tested the brake pedal switch. I do have brake lights working, so that wouldn't appear to be the issue. Any thoughts from you guys?

Did you disconnect the battery while doing the work?

Most likely culprit is sensor plug on the rear wheels but if you say you didn't disturb those then very odd or coincidental timing. The sensor needs to see those slots or grooves on the wheel hub / spline.

No I did not...

After this problem cropped up, I inspected the sensors (which are the companion sensor to the brake wear sensors plugged into that clip/receptacle behind the wheel hub, right?) they all seemed fine. I checked them to make sure they are seated.

So this problem fixed itself. I put the wheels back on and took it out for a spin. I didn't go 100 ft and the ABS light went off. Has stayed off. Maybe after doing the brakes and spinning the axles without the wheels on, the sensors needed something to detect?? Anyway, glad it was resolved for a very low price.

The sensors detect the teeth or notches in the hub. No need for tires. Maybe grease or dirt over a sensor end or something bridging some notches so it couldn't see them properly. Or connector seated and started reading.

Glad that worked out easy.