O2 sensor

https://www.2carpros.com/articles/volvo-code-definitions-and-code-retrieval-procedure-1983-to-1993

Most cars 1995 and older typically had about two dozen codes they could set. 1996 and newer models use the "on-board diagnostic system, version 2", (OBD2) emissions system. Those have well over 2000 potential fault codes. About half of those codes relate to something that could adversely affect emissions. Those are the codes that turn on the Check Engine light.

Your car is somewhere in between these two extremes. You do not have the OBD2 system, but what you have is more sophisticated than most other 1995 models.