Lean flammability limits (LFL) of syngas mixtures were measured at different levels of inert dilution and unburned gas preheating temperatures using a counter-flow flame burner. LFL increases drastically when inert dilution (N-2 + CO2) exceeds a critical value, at which the extinction flame temperature also increases remarkably. The abrupt change in LFL at high inert dilution cannot be predicted by Le Chatelier's Rule, which is derived on heat balance basis. This suggests that thermal quenching is insufficient to capture the influence of inert dilution on flame extinction. Data analysis demonstrates that the dependence of LFL on inert dilution can be well captured by a flame temperature chemical time correlation, revealing thermal-kinetics coupled effects of inert dilution on flame extinction. In contrast, LFL decreases linearly with preheating temperature, resulting in a constant extinction flame temperature, which indicates only thermal effect of preheating on flame extinction. The contrast suggests distinctive effects of inert dilution and preheating on flame extinction. Both thermal and kinetics effects of inert dilution need to be incorporated for precise prediction of LFL of syngas at high inert dilution, whereas only thermal effect needs to be considered for unburned gas preheating.