Tested system not heating for the main level of the home. Found system has a failed gas valve that is not opening. Customer also stated that they regularly have to turn the system off and turn it back on to hard reset the system in order for it to heat. Noticed that this was due to overheating while the furnace was running because the system is lacking return air. While I was here, I inspected the heat exchanger and found that there is separation at the joint where the tubes of the heat exchanger meet the inducer chamber. Based on our findings today, it might be time to think about getting a brand new system.