We have a Goodman GKS9 Gas-Fired Warm Air Furnace. I've changed the air filter, cleaned the flame sensor, and checked the condensation pipes for clogs. They're clear. The behavior seems strange to me. If I bail out all the water from the condensation trap (stick in a paper towel, squeeze it out, repeat until the trap is empty) and then set the temperature to high (around 80), the furnace will keep running without any issue--EVEN AFTER the trap fills back up. However, if I stop it and try to turn it back on, it runs for about 4 minutes and then stops. The red light on the circuit board flashes 3 times. The manual says

Fault description: * Pressure switch circuit not closed. * Induced draft blower is operating

Possible Causes: * Pressure switch hose blocked pinched or connected improperly. * Blocked flue and/or inlet air pipe, blocked drain system, or weak induced draft blower. * Incorrect pressure switch setpoint or malfunctioning switch contacts * Loose or improperly connected wiring

Corrective Actions: * Inspect pressure switch hose. Repair, if necessary. * Inspect flue and/or inlet air piping for blockage, proper length, elbows and termination. Check drain system. Correct as necessary * Correct pressure switch setpoint or contact motion * Tighten or correct wiring connection.

Could it just be the pressure switch itself or is there a more likely explanation? It seems strange to me that it will keep running if I initially start it with an empty condensation trap and continue running even after the trap fills back up.

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