One controversial issue associated with enhanced geothermal systems is induced seismicity, which limits their broader application. As induced seismicity has a strong correlation with the activation of pre-existing faults, understanding processes controlling the potential fault activation during geothermal heat extraction is of critical importance. In this study, a coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method to investigate the triggering mechanisms of potential fault activation during geothermal heat extraction. The developed thermal and hydraulic solving frameworks account for the heat transfer of various kinds and fracture fluid migration in the rock mass, respectively. The fully coupled THM scheme is established by pairwise coupling between thermal, hydraulic, and mechanical solving frameworks, and is then progressively verified against analytical solutions by five validation examples. Finally, cold-fluid injection-induced potential activation of a fault during geothermal heat extraction is investigated. The results demonstrate that the gradual aseismic opening and slip of the fault at low injection pressure are caused by the thermal contraction-induced reduction of the imposed normal stress on the fault surface due to convective cooling. The thermally induced aseismic slip weakens the fault and contributes to triggering sudden (unstable) fault slips. The convective thermal transfer coefficient is found to greatly affect the onset time of the fault aseismic slip. The effects of the injection pressure and temperature as well as the in-situ stress field are also discussed. The findings justify thermal effects in controlling the potential fault slip behavior and illuminate the triggering mechanism of unexpected seismic activities during geothermal heat extraction even at injection pressure below the safety threshold value. Highlights A fully coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method (FDEM) to investigate the potential fault activation during geothermal heat extraction. Fault aseismic slip caused by thermal contraction due to convective cooling at low injection pressure contributes to triggering unstable fault slip. The convective thermal transfer coefficient greatly affects the onset time of the fault aseismic slip. The injection pressure affects the time at which the fault aseismic slip becomes stable and the in-situ stress heavily affects the aseismic slip amount. Thermal effects are justified in triggering seismic activities during geothermal heat extraction at injection pressure below the safety threshold.