2-4 star Hotel with A/C near Gare Du Nord in Paris

There is at least one Ibis in the neighbourhood.

I saw 2 ibis available nearby, but one reviewer commented the a/c in an ibis had broken. Do you consider ibis a good hotel chain?

For one night the Ibis is perfectly fine notwithstanding the AC problem. We stayed one year at Hotel Caravelle on rue des Martyrs, walking distance from Gare du Nord. Have a look at that one.

I stayed in the Mercure Paris Terminus Nord recently. Directly facing the Gare Du Nord so highly convenient.
Website: http://www.mercure.com/gb/hotel-2761-mercure-paris-terminus-nord/index.shtml

It had vicious air conditioning in the room. It displayed, and felt like, a temperature of 21°C even though it was over 30° outside.

The hotel is currently being rebuilt, so only part is open. It hasn't got a restaurant, and breakfast is not included, but the hotel occupies the upper floors only, and on the ground floor and in surrounding streets are many restauarants some offering breakfast.

If you look on hotels. com, you can filter for the number of stars and the neighborhood. I just looked for the first full week of August, and there were quite a few. Then cross check with TripAdvisor (and here) for reviews of hotels you're interested in.

An Ibis is what an Ibis is.

Same chain group as Novotels, Suite Novotels, Ibis Budget, Mercure, Ibis Styles, M, and many more - all part of the Accor group. Certainly the biggest chain in France, also the biggest hotel groups, and probably all of Europe. I haven't counted but I bet there are 2 or 3 dozen Ibis in the Paris area.

1 night around gare du Nord, hmmm. One night ok but I wouldn't want to make a habit of it.

Well, that's the thing, Nigel. It is for one night, one single night and so convenient for the train in the morning. If the OP stays there, he needn't, unless he wants to, even walk about in the neighbourhood which isn't all that bad after all.

And for the Ibis, might as well join the Accor club (free) which gives about 10% off rack rates.