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.