The heater doesn't blow hot air?

I recently broke my knob off too & have been using pliers to switch it back & forth from cold to hot. I dont have your current problem of luke warm air but 1st use pliers & make sure its all the way over to hot. If thats not the issue, make sure you have plenty of coolant (antifreeze). If not that its 90% likely you need a new thermostat. If money is an issue for you like it is me, go to a scrap yard & get one. (call around 1st) If you cant swap it out yourself or know someone who can, its cheapest to first contact mechanics on Craigslist 1st.(this is what I do for lots of stuff; there are a lot of great "trade" workers with a side job) If thats not your thing contact local shops for prices & avoid dealerships if price is important to you. Good Luck!