The other time the ABS/ETS threw yellow lights on my 2000 E-320, it turned out to be a failing lateral motion sensor located on the automatic transmission. When the yellow lights came on and stayed that way, I took it to a MB dealer, who read the codes and replaced the sensor. Two years have passed and I haven't had any repeats. When the problem was developing, the yellow lights came on at seemingly random times, but always when the car was moving under power, and usually followed a quick change of direction. By all means, get those codes checked out.