Can a MOT tester sign a passport?

No, a MOT tester cannot sign a passport as a witness. The UK passport application requires a countersignature from someone who has known the applicant for at least two years and is in a position of good standing, such as a professional or a person of good repute. While a MOT tester may hold a relevant professional qualification, they do not typically meet the criteria for passport countersignatories unless they also qualify under other criteria.