A diploma or GED is not required for ASE (National Institute of Automotive Service Excellence) certification, but it will make things a lot easier.
Successful completion of automotive technical courses in high school is recommended but not required. Before registering for the certification test, applicants must have experience that may include on-the-job training and/or certification preparation classes. Without a diploma or GED, on-the-job training may take longer to accomplish since a job may be harder to obtain and the starting position may be lower.
Diploma/GED requirements for trade schools vary from school to school. A good resource to find one that accepts applicants without a GED is the local trade union. They should be experts on the local training and certification options.
To become a Certified Welder through AWS (American Welding Society) there is no requirement for a diploma or GED. AWS, one of the largest welding certifiers administers a strictly ability-based certification test. If you can pass the test, previous training and education is irrelevant to the certification process.