Detecting the magnetic field generated by a coil-on-plug (101) (aka coil-over-plug) using an adjustable probe (111). The probe (111) includes a signal detector (115) for detecting the field, a detachable adapter (117) configured to detachably attach to the housing (107) of the ignition coil at a specified location (113), and an adjustable connector (119 and 121) that adjustably connects the signal detector to the detachable adapter in a first predetermined position or in a second predetermined position. A detecting process and a diagnostic system using the probe are also disclosed.