What is an EPROM?

An EPROM is an electrically programmable read-only memory, or a type of computer memory that can be programmed only once, but read an unlimited number of times. EPROM ECUs have such a memory IC installed in them. This memory holds the program code that controls the ECU behavior. The EPROM is a separate chip from the microcontroller (computer chip) that actually runs the program. This makes it easy to change the ECU programming, since only the EPROM need be replaced, and EPROMs and their associated programming tools are relatively common.

The alternative, a non-EPROM ECU, does not have an EPROM. Instead, the program is stored inside the microcontroller itself (technically, in embedded EPROM memory that is part of the microcontroller). This makes it hard to change the ECU programming, because the microcontroller needs to be replaced. These are difficult to get, and require special programming tools that are equally difficult to find.