Like Bill said, a 1-5-5 code on port 3 (ABS) of the A diagnostic connector is a faulty signal from the rear axle speed sensor (on the case of the differential).

Is the speedometer working okay? That rear speed sensor signal is shared by both the speedometer and the ABS controller.

If the speedometer isn't working then there's possibly a broken wire (likely in the exposed area around the axle) or an externally damaged rear sensor. If you've recently been working on the car then think about any wiring or connectors you may have disturbed, like under the dash.

If the speedometer seems to be working okay then it's quite possible there's an intermittent fault, most likely at the rear. The rear sensors themselves are generally extremely reliable and don't need cleaning. There's little point in removing it at this stage, but do carefully check it for signs of damage or cracks as well as carefully inspecting the wiring and wire harness underneath. If there's nothing obvious then maybe check near the inside where it goes through the body. The little flat anti-tamper box is an unlikely problem area.

If an inspection underneath doesn't reveal anything then clear the diagnostic code and see if it comes back. If the ABS light comes back on right away then there's a problem you'll want to try isolating with a meter, like looking for an open circuit, possibly needing to open the ECU and/or ABS connectors for probing. If the ABS light later illuminates, like going on rough road or over a speed bump or pothole, then you know there's an intermittent connection somewhere, something's loose or wiring is damaged. That will require a bit more finesse to isolate.

See the Brakes ABS section of the FAQ here
https://www.brickboard.com/FAQ/700-900/BrakesABS.htm
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now