Your heater is not working right and you suspect you have a bad heater core. If you arrived at this conclusion based on finding the typical symptoms of a bad core, then you are probably right. On the other hand, if you've never diagnosed a bad heater core before, you may need some guidance from experts who do this every day. So before you jump into the job of replacing your heater core, we suggest you spend a few minutes watching our training videos.
A few minutes spent here might save you many hours of work, only to discover that it was not the heater core after all. Here's a word of advice: If you have to do the labor of getting to the heater core, only to find it isn't a bad heater core after all, just go ahead and replace it anyway. The core is cheap compared with the effort required to replace it. You don't want to put it all back together and then have to do the job again a month later when the old core fails.
An additional way to determine whether your cooling system is leaking is to have the cooling system pressure tested. A system that is not leaking will maintain the pressure. If there is a leak anywhere in the system, the pressure will fall over a period of time. It is not likely that you will have a cooling system pressure tester unless you are (or used to be) an automotive technician. However, most repair shops will pressure test your system for little or no cost. So, if you are having trouble determining whether you have a leak, a pressure test is definitely the next step before digging for that bad heater core. Here's more about the pressure test
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