Mercedes R Klasse Diesel ABS and Steering Issues

Sounds like more than just a slightly low tire. When a tire goes flat, or experiences significant pressure loss, it can cause all sorts of knock-on effects. The tire monitoring system is obviously picking up the pressure difference, which triggers the initial warning. The ABS light and steering issues suggest the car's stability control systems are also getting confused by the difference in wheel speeds. Do you have tire pressure monitoring, also known as TPMS? If so, it could be a faulty valve in one of your tires or a punctured tire causing the pressure loss.

TPMS... I think so. What does a faulty valve have to do with the power steering?

The power steering itself might not be directly affected by the valve, it's more that the flat tire is causing the system to work overtime. Imagine trying to turn the steering wheel with a completely flat tire – it's going to be much harder. The TPMS valve is just a likely entry point of air loss. The punctured tire is the main culprit here. I would suggest taking it to a workshop to get it checked out.

Thanks for the insights. Took it to the shop, and you were spot on. Defective valve in the tire. New valve and re-pressurize the tires and all seems to be working well now. Total cost was 280€.