Ignition of non–premixed boundary–layer flows over a hot wedge–type porous wall with fuel injection is analyzed mathematically and numerically. The free–stream fluid is a diluted oxidizer such as air. An ignition criterion corresponding to the condition for lower vertical tangency on an S-shaped ignition–extinction response curve is derived by the method of large activation energy asymptotics. This ignition criterion depends on all relevant system parameters such as flow conditions, thermophysical properties (Pr, SCoand Scf) and the overall chemical kinetic mechanism of reactants. The relative sensitivity of system parameters affecting the ignition is investigated. In particular, the influence of system parameters on the transition from the kinetically controlled to the diffusionally controlled ignition mechanism is studied. Numerical calculations are also performed to compare with the asymptotic results. The asymptotic solutions are found to be in good qualitative agreement with numerical solutions.