Technician Ricardo performed prepaid preventative heat check on a Lennox closet gas furnace, changed out the16x25x1 air filter can use a 16x25x4. checked for carbon monoxide leaks & didn't find any checked for any electrical or heating components that may need to be replaced everything checked just fine no issues found. extra filter left behind, recommendations replace 30x10 return grill other than that system is ready for the cold winter