Whilst basil pesto is low on carbs and accetable on a LCHF diet, tomato sauces are a whole different story. They are usually laden with sugar, much to much sugar for a low carb diet.
But it's not impossible to find a suitable tomato sauce. If you live here in Sweden, Saltå Kvarn's Pastasås Napoletana has only 5g of carbs. For a tomato sauce that's awesome. It's also organic (ekologisk). So my tip for you is make sure you read the labels.