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So this is a portion of the stimulus that contains conditional reasoning and when drawing out my sufficient and necessary conditions diagrammed a mistaken reversal.

"Therefore, the only effective way to reduce such emissions is to replace the conventional diesel fuel and gasoline used in automobiles with cleaner burning fuels such as methanol that create fewer emissions."

When I diagrammed this my diagram looked like this:
Replace-->Effectively Reduce Emissions

While the correct diagram should have been:
ERE-->Replace

The presence of the "only" before effective way to reduce... tripped me up to make me think that it was the necessary conditions.

Any advice on how this type of error could be avoided?

"The only" introduces sufficient. "Only" introduces necessary.

"the ONLY effective WAY to reduce such emissions is to replace the conventional diesel fuel and gasoline used in automobiles with cleaner burning fuels such as methanol that create fewer emissions."

"Only" does introduce the necessary in the conditional.
But, in this case, the necessary introduced is the "way."
So ask yourself: what way is used to reduce emissions

A rephrase:
If you want to effectively reduce emissions, then you must replace the conventional diesel fuel and gasoline...

That's just how I personally look at the question.

The phrase "the only" introduces a sufficient condition, whereas the phrase "only" on its own would indicate a necessary condition. That is why the correct diagramming is the opposite of what you originally though. Just remember the difference between "only" and "the only" and you will be fine.

I didn't know that about "the only" that will definitely help me out now. Thanks guys.