My evco flashes 3 green lights saying it's a combustion air switch is not closed I replaced it but get same 3 green flashes.

The combustion air switch may not be closing because the combustion fan is not running. Is that the case? You could have checked the old switch with a meter to see if it was bad. You will need to get a good meter and learn to use it before you will be able to fix this unit. Guessing and just changing parts will never work and never has. The combustion air fan has a switch to turn it on, so either that switch is not closing or the fan motor is bad. If so you usually need to replace both the motor and the fan because the fan rusts and comes apart from all of the acid exposure. DrTom4444