On a no heat call today. Furnace locking out on high limit. Went to check filter and found none. Checked evaporator coil, and turns out, its slightly dirty!!! Homeowner has lived there seven years and never knew about a filter. Worst part is that someone was out last month. And replaced the flame sensor. And a year ago someone found a "problem with the gas pressure"!!! When i pumped the system down to remove coil, i got a look at the condenser coil! First pic is evap, second is condenser. Btw, they have dogs.