Code PO302

Without testing it could be many things. Bad new parts are unfortunately very common, next it could be a bad fuel injector or a bad control or power control circuit to the coil or injector. It could be a bad head gasket or it could be low compression. If they tested the compression and it was much lower than the other cylinders then the mechanic could be correct. Compression testing isn't that hard. You can get the tester as a loaner tool at many parts stores, then you remove the spark plugs and remove the fuse or relay for the fuel pump and test the compression, You want all of the cylinders to be within 10 percent or better from high to low. Say you check cylinders 1 and 3 and find 195 and 200 psi but cylinder 2 shows 90 psi, you have low compression in that cylinder. That can be from bad rings or damaged cylinder or bad valves.
https://www.2carpros.com/articles/how-to-test-engine-compression

I wouldn't say ignorant, just not your field of study. These days everyone can use some help at times, especially with the costs of repairs going way up and some places thinking they can make "easy money". If we helped in any way, great. Thank you for using 2CarPros, please let us know what you find or decide to do.