This is the most crucial question for ensuring engine reliability and longevity.
Genuine Cummins Parts: These are the gold standard. They are designed, engineered, and tested by Cummins Inc. themselves. Purchased through an authorized Cummins distributor or dealer, they come with a full warranty and a guaranteed perfect fit and performance. You are paying for absolute certainty and quality assurance directly from the original manufacturer.
OEM (Original Equipment Manufacturer) Parts: In many cases, this is the same as a Genuine part. True OEM parts are made by Cummins or by a contract manufacturer explicitly authorized by Cummins. They meet all original specifications. However, the term is sometimes used more loosely, so it's important to buy from a reputable source.
Aftermarket Parts: These are produced by third-party companies not affiliated with Cummins. Quality can vary dramatically—from excellent to subpar. While they are often less expensive upfront, they may not have the same rigorous testing, could affect performance, and might even void your engine's warranty. The risk of premature failure is higher.