Bosch Dishwasher won't drain

[dj928]

My Bosch dishwasher suddenly stopped draining. I checked the fine filter that leads to the drain inside the dishwasher and it was clean. I rinsed it anyway. When it apparently tries to drain, I hear a sound from the airgap and a little water drains into the sink drain (by the disposal). It tries about 3 times and then seems to give up. I don't know how to check other areas where there might be blockage. Thanks.