Once an engine gets old it can fail for any number of reasons. Perhaps it suffered from too much heat exposure over time which caused the metal to expand and the engine to eventually fail. Maybe the cylinder heads cracked or warped or maybe an EGR valve caused detonation damage. Either way, once they come to the end of their life spans, unless you plan on buying new or used cars to replace them, failed engines must either be rebuilt or replaced by used engines.
For the environmentally conscious human being, this poses a difficult question. Is it greener to just remanufacture your old engines, or is it better to have them replaced by used engines? As is well known, the used engines option is the safer one. Used engines have already been broken in, are factory tested and inspected for flaws. Used engines are also not as prone to a mechanic's possible mistakes as remanufactured ones are. But are used engines better for the environment than remanufactured ones? What about buying a whole new car?
It's easy to see how buying a new car would be the less green option. Buying a new car when your engine fails involves leaving the failed one in a junkyard and consuming tons and tons of new materials, all of which have to come from somewhere and then go somewhere else in a few years. Although the new car you might have bought has already been made and the materials already used and consumed, your act of buying it creates a slight spike in the demand for new cars which use new materials. Multiplied many times over by all the decisions made to buy new cars and discard old ones every minute and the spike shoots off the charts causing a high demand in the market for new cars. This process slowly but surely eats away at our precious environment and resources.
Buying remanufactured engines instead of used engines creates the same process, though on a smaller scale. Rebuilding an engine involves buying many new parts to replace the retired and failed ones in your engine. New materials are used, resources consumed, and recycling processes ignored. Not only will rebuilding your engine involve an investment of money into a process that is widely considered a serious risk and highly prone to human error, but that process is also a non-recycling, not environmentally friendly one. On top of all that, your engine, once remanufactured, may develop the same problems that caused it to fail in the first place, and all the new parts you just invested in putting in could suffer the same damage as the old ones and cause you to have to begin the process from the beginning all over again.
Used engines, on the other hand, are dismantled professionally from their former vehicles; their fluids are carefully removed and, in many cases, recycled. All useable parts of the used engines are conscientiously prepared for their new owners and all non-useable parts of their previous host vehicles are recycled into reusable metal chunks.
When you choose the used engines option, no new materials are used. Nothing new is taken from the environment to get your car back in running shape, and no new demands are placed on our precious resources. Not only are they a cheaper, less stressful, and quicker option than remanufacturing your engines or buying new cars.