Is this a 1.6 petrol or diesel?
Ok typically, on these engines and with the fault you describe the fault is lrelated to the DPF filter.
The DPF stands for diesel patriculate filter. This in its basic form is a sut filter.
When these run out of fluid, or if the DPF itself hasnt done its self clean cycle, then the car will enter limp home, or limited function mode.
Unfortuantly this will have to be done by Citroen.