I have a Goodman GMP125-5 that runs all the time even with the thermostat off and the fan to auto. The only way to stop it is to turn off the power to the unit. I have done the following.

Circuit board LED blinking 4 times so I tested the overlimit switches. All are in closed and in testing the overlimit circuit from the board to the gas valve it showed a closed circuit on my VOM.

Tested the pressure valve by turning the power to the unit on so the inducer motor would start and the pressure value switch closed. I assume this switch is "open" when the motor is off and "closed" once the inducer motor comes on and creates enough pressure on the valve.

I feel I have eliminated both the overlimit safety (closed circuit) and the pressure switch (closes once the inducer motor is on) as issues so why do I still get 4 flashes on the LED?

Any ides on what I should check next? Is there any way to test the circuit board?