Generally speaking, a repair shop will warranty or guarantee the specific work they performed for a certain period of time. That being said, a head gasket seals the cylinder head to the engine block. The cylinder head could be considered the top of the engine while the engine block is the lower part. When replacing a head gasket, it is general practice to inspect both the head and the block for damaged sealing surfaces and for excessive warpage. If they failed to perform any of these steps properly, it could result in a repeat head gasket failure in the near future. If the same part failed that they just installed, ideally should stand behind their work.
Below is an article that explains how a head gasket works and demonstrates various methods to test for a failed gasket.
https://www.2carpros.com/articles/head-gasket-blown-test
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Monday, November 4th, 2019 AT 6:45 PM