2005 Chevrolet Express Engine Rebuild Fiasco

There's no easy way to check an engine outside of the vehicle. Also, fault code 300 means a random cylinder misfire. There's a number of potential causes for that and none of them have to be related to previous or future repairs. In this case it might pay to get a second opinion from a different shop. Don't tell them what to look for, meaning what someone else claims to have found. Just give them the symptoms and let them come up with their own diagnosis.

Just like when a doctor needs to know your history, it's helpful for the mechanic to know your car's history too so it's okay to tell them this is a different engine, but it's also important they know it ran okay for many months. That proves the previous repair was done properly and this new problem is most likely not related.

If they ask, it's okay too to tell them this current problem was already diagnosed by someone else, but don't tell them what that diagnosis is. The reason for that is you don't want them to take shortcuts or to overlook something. If you tell them it has been diagnosed as a leaking cylinder head gasket, the new mechanic will be inclined to go straight for that to verify it, and stop there. Time is money and they want to complete their diagnosis as quickly as possible. If he has no reason to look further, he may stop before finding other things or different things that are needed.

To further complicate matters, if you tell them who already diagnosed this problem, and they're friends with that shop, they may tend to agree with that diagnosis, even if it's wrong. A much simpler repair may be needed but they'll replace the head gaskets too, just to make their friend look good.

If the two shops are not on friendly terms, the second mechanic is going to try to make the first mechanic look incompetent. Doing that never helps anyone's reputation. If the first shop is less than reputable, and the second one knows it, that second mechanic knows he doesn't have to cut the first one down. His quality work will speak for itself. Also, he should be able to explain the tests he performed and how he diagnosed the cause of the problem. Mechanics normally don't have real good communication skills, and, every minute they spend talking with you is time lost working on another customer's car, but experienced mechanics who are confident in their diagnosis, will always find a few minutes to explain what they did and what repairs are needed.

If the second mechanic comes up with a diagnosis other than head gaskets, THEN you might want to share the previous diagnosis, then let the new guy explain why that is incorrect. It's also possible for the chemical test for a leaking head gasket to come up negative under certain conditions, and the test may need to be repeated. There's a bunch of potential causes for the fault code 300. A leaking head gasket can cause that but there's lots of less-serious causes too.