2016 Citroen Xsara Picasso it can be running fine then all of a sudden u can’t rev over 1500 and i had to stay in 2nd gear to get home

It can be running fine then all of a sudden u can’t rev over 1500 and I had to stay in 2nd gear to get home. Nothing shows up on diagnostic test. The car has only done 36k. We have changed the air filter twice. Car was in garage for 4 weeks they thought they solved it by changing an injector in ad blu tank, car run lovely for few weeks, filled up with diesel and 2 days later same happened so back in garage. No warning lights on