I have a Goodman GMP125-5 furnace. A year ago, my circuit board was replaced by a repairman. He also replaced the heat exchanger, because he said it was cracked. It worked great until about a month ago. The vent fan would come on, then the igniter would light, but no gas would flow. After 4 attempts from the igniter, it would go into lock out and try again later. Sometimes it would work normally and heat the house. Then it would go back to no gas flow. The repairman came back and said he cleaned the tube to the vent switch. The furnace worked again normally until today. Its doing what it did a month ago, sometimes the gas will not flow.