You start by troubleshooting to determine exactly what the problem is. There's more than one thing which can go wrong with a heater, so you either diagnose it to narrow down the cause, or you just throw parts at it until you get it right.
In order to probably repair the heater core in your 2000 New Beetle, you will probably have to get the entire heater core replaced.
no no
The heater core is underneath the dash in the middle section right behind the console. You can fix it by removing the dash.
is your heater core bad or thermostat bad need more details
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The only fix for a leaking heater core is to replace it.
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The only fix for a leaking heater core is to replace it.
find out what is wrong and then correct it. if you ask vague questions you are going to get vague answers.
You can replace the heater fan motor .......remove glove box, four screws hold it in.......then remove screws from motor and drop it down out of the housing.
No way to fix that - your tank is scrap.
Not practical to try to fix a heater core. Replace it with a new one.