Amber light after Cat change

I had to replace Cat after it was stolen. All was fine until I took car on motorway and amber engine came on. Took it for diagnostics to be on safe side. Toyota said it was fine, sensors were fine. But just came up as a non familiar Cat. Meaning as it was not a Toyota Cat it did not like it. They cancelled the code and the light went off they said they could not guarantee that light would not come on again. The light has come back on again after motorway drive.

Question is, do I just have to put up with the light coming on- could it be the wrong or a Bad Cat?

I have bought a OBD2 to have a look myself.
But have people had this experience with fitting such parts?

It’s a 2008 Prius so the £2,000 plus to get Toyota to replace it seemed too much. At the moment it’s cost about £700 including changes sensors and the diagnostics