The brief
The B180 had its ABS and traction control warning lights on, and the speedometer was behaving erratically, jumping around or reading wrong. He brought it in. Those symptoms point at a wheel speed sensor. Each wheel has a small sensor that reads how fast it's turning, and the car uses those readings for the ABS, the traction control, and on these the speedometer too. When one of those sensors fails, the car gets a bad signal, so it puts up the ABS and traction warnings and steps those systems back, and the speedo readout goes haywire. The brakes themselves still work, but a failed sensor doesn't recover, so it needs the sensor changed.
The diagnosis
On the scanner the codes pointed cleanly at one wheel speed sensor, its signal dropping out, and a meter check at the connector confirmed the sensor itself was the failed component, with the harness and the tone ring it reads off both fine. That's a sensor replacement. A failed wheel speed sensor doesn't come back, so the sensor was getting changed.
The work
The car went up, the failed wheel speed sensor was unbolted from the hub and a new genuine Mercedes-spec sensor fitted with a fresh O-ring, the harness clipped back into its proper run so nothing rubs. Then the stored faults were cleared and the system checked over. A road test confirmed the warnings stayed off and the speedometer read correctly.
The outcome
ABS and traction control warnings off, the speedometer reading correctly, and no codes after a drive cycle. The B180 went home with the safety systems and the instruments working properly again. A failed wheel speed sensor leaves the ABS and traction control offline, so getting it changed put the safety net back where it should be.