Almost any experienced mechanic (this is not a Volvo issue) knows that THE number one reason for the Check Engine light coming on, especially intermittantly, is a tired or contaminated O2 sensor. Number two is a failed coolant temperature sensor but that usually causes extreme flooding, no-start or black smoke.

In short, the O2 sensor was many times more likely to be the correct answer - even if it was a Chevy or a Toyota.

At the other end... with the exception of a few specific ECUs (Volvo has one version in the 700 car that is known for fuel pump circuit failure causing no-start) or damage due to improper service and testing... as often as it might get replaced, the ECU is rarely the actual cause of emission failure, poor running or check engine lights. This is true for American, European, and Japanese cars.