This article examines the generation of hot-spots and their impact in the heterogeneous Suzuki–Miyaura coupling reaction for the synthesis of 4-methylbiphenyl in toluene solvent in the presence of Pd/AC. Hot-spots easily formed on the surface of the activated carbon (AC) catalyst support under high electric field conditions; they were recorded in real time using a high-speed camera. Chemical yields of 4-methylbiphenyl under the microwaves’ magnetic field (H-field) were 2-fold greater than those under electric field (E-field) conditions at identical temperatures and reaction times (120 min). Microwave E-field irradiation with a high accuracy device enhanced generation of hot-spots. Excessive formation of hot-spots impacted negatively on this coupling reaction.