2001 Fiat Ulysse ENIGINE LIGHT COMMINING ON AND OFF

Hi my partner has a Ulyesse we have had for a couple of years. The engine light came on then went off. Now it comes on for days on end or 5 min but when it dose the car goes into like a safe mode you cant go above 2500 rpm. I have been looking for a fault code reader but cannot find one that is suitable for diesel engines pre 2004. The uylesse has the universal socket in the drivers foot well. But all the reads I have tide say there is no fault.

so if any one knows of this problem or a fault code reader that works on this engine I would be great full for the help.