The sensor voltage of the O2 sensor is very small by comparison to other sensor readings. In many instances a .2v voltage drop is inconsequential. In a O2 sensor it is s deal breaker. An O2 sensor voltage of .5v needs to reference the same ground as the control unit. One of the surest ways to do that is make real sure both are referenced to the same thing: chassis ground.

In newer 4 wire O2 systems the ground for the sensor is brought back to the control unit to assure they are the same. I presume someone was just making sure.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician