Hiya i have a peugot 307 1.4 hdi 03 reg?

i have a peugot 1.4 hdi 03 reg and the antipollution fault is on all the time, i have only had the car just over 2 months and it has only done 40-000 miles, just can't understand it, the garage has said it needs topping up witha cetain fluid, could anyone tell me what this fluid is please?