When I arrived, the furnace was in a one hour lockout due to loss of flame. I reset the furnace and made the system restart multiple times. During one of the startups, the gas valve was delayed in opening its valve. Technician from before had the same issue. I double checked voltage to the gas valve from the main control board and the board was sending proper voltage to the gas valve. This left the gas valve being the culprit as to why the system was not heating properly. With customer approval, we replaced the gas valve under part warranty. I made the furnace restart multiple times and furnace is operating properly at departure. Spoke with customer about the type of filter they were using and how restrictive they are on airflow. Customer will purchase new filters that are less restrictive. I recommended using filters with the merv rating between 8 to 11.