The service engine light was on and took car to Ford dealer. $515 to replace dpse(?) sensor. Light is out but told engine has a lot of carbon buildup and $400 to perform work. I was told earlier by another mechanic that #5 cylinder is not firing. Is all this connected? Had cat converter replaced by Ford at 68,000 miles. Should I just trade it off?

How you getting a carbon build up? Take a fast 75 - 100 mile trip, good for carbon clean out. Don't sit and let your engine idle needlessly.