What do you always check before buying used diesel engines?

Used diesel engines can save money, but they can also turn into a headache fast.

For those who’ve done this before:

Do you ask for testing details?

Is a warranty a deal-breaker?

Do you trust nationwide sellers as much as local yards?

I’ve been comparing a few options, including places like NationwideHeavyTruckParts, but I’m curious what actually matters most in real-world experience.