A common problem with emission test failures is that guys just start throwing expensive parts at the problem. This could be the owner or a mechanic.

M103 engines are typically at the ragged edge of the HC limit on the CA 15 MPH ASM test. It's the nature of the beast.

Troubleshooting should begin with analyzing the test result numbers, particularly the amount of O2. If it's more than a tenth or two percent, it means the converter was not hot enough. This can be mitigated by properly "managing" your test as I've posted a zillion times.

Any modern car can be a money pit if you just throw parts at it helter skelter, without doing diagnostics and analysis.

Duke