I had some repairs done on my car this fall and I mentioned that my heat wasn’t working properly. They told me someone had used stop-leak in my car, and that it had ruined my heater core. They quoted me $1100. I have not had the money to repair it until now. I called several area mechanics and three different places said it is very strange that I would need a new heater core, and they recommended taking a look at it to see if there is a cheaper alternative. How likely is it that I need a new heater core?
I doubt stop leak ruined your heater core, Honestly we are not able to see the car and perform any tests, but a mechanic that can tell you why you have no heat by verifying the condition will do you better in the long run. I mean, no heat, bad heater core, easy first guess, but maybe the heater core is good and it is a control valve instead. How likely it is you need a new heater core is best determined by visual inspections and tests, not an internet diagnosis, I don’t mean to sound cruel but you need a qualified person to inspect your system and make a proper diagnosis.