The Nyquist impedance spectra of proton exchange membrane fuel cells fed with air exhibit generally two loops, the low frequency one being ordinarily associated with oxygen diffusion. However, as shown experimentally by Schneider et al. and numerically by Kramer et al., the convective motion of air along the flow field channels results in an additional impedance that contributes significantly to this low frequency loop. In this paper, we derive an analytical expression of a 2D convecto-diffusive impedance taking into account convection along the channel direction, which makes it possible to determine the extent to which the low frequency loop results from diffusion losses in the direction perpendicular to the MEA or from convective phenomena occurring along the channel direction. This work confirms that AC induced sinusoidal variations of the oxygen concentrations amplify significantly along the air channel, especially at low frequency. However, we show in addition that using constant air (and hydrogen) flow rates when performing EIS contributes also significantly to the low frequency loop: since the air flow rate is not oscillating in phase with the current, the oxygen stoichiometry is not constant and the operating conditions in the presence of the AC signal cannot converge to the steady state conditions when the frequency tends to zero. As a consequence, contrary to the common conception, the fuel cell low frequency resistance R_lf cannot be equal to the slope of the polarization curve R_DC (in identical operating conditions). The bias between R_lf and R_DC is all the more significant since the oxygen stoichiometry is low; locally, it is the most important near the air channel outlet. In order to assess the magnitude of these effects, it is possible to use a non-dimensional number, function of the characteristic times associated with convection along the channel and with diffusion in the direction perpendicular to the membrane.