Without seeing the model, I'll have to guess:
1) you are not supplying enough air to the cathode. The current will dictate the O2 (trace gas on the cathode side) consumption rate and once it's all consumed, you will get this error.
2) your initial condition didn't have enough O2 in there. Maybe the supply will soon pick up, but the amount of O2 initially wasn't enough so you run out early.
Could you please check how you specify or control the air flow into the cathode channel, and whether the initial conditions in the channel make sense?
