40th Anniversary Convertible 6 Cylinder Question

Hi all! I am brand new to this forum and this is my first post so I apologize in advance if this question has already been beaten to death: my son just got a 40th anniversary convertible 6 cylinder Mustang. When we got it it ran like a weak 4 cylinder, the check engine light was always on, and it drank gas like nothing I've ever seen. We did a complete engine flush at a Ford dealer, checked the code which turned out to be an oxygen sensor and replaced that. Now it runs like a weak 6 cylinder, and the check engine light is still on (same code) and it still drinks gas like crazy. I haven't checked the mpg yet but I suspect it's no better than 15 mpg. SOOO... I thought I'd pull a spark plug to see if they needed replaced. Those buggers are hard to get to so of course I reached for the closest one and I pulled, and I pulled, and I yanked on the boot so hard I'm sure I was just about to break it apart. Would. Not. Budge. Not even a little. Any advice on how what I'm doing wrong in trying to get the boot off the plug? Also, on why the engine is so weak and the check engine light is always on? (BTW - the Ford dealer told me that I need a new CPU, which is supposed to be a diagnosis of exclusion after checking a list of about 10 other possible issues, but they just skipped those steps and gave me the most expensive option. With no guarantee that it would turn out to be the actual problem.) Sorry for he long post! Any help/comments would be greatly appreciated! Thank you!

Beuller? Beuller? ANYWHO...I have since found out that the car is a 3.9 liter, 17mpg city, 24 highway and 193 horsepower. I don't understand why anyone would manufacture such an engine - where's the value? I replaced the wires and plugs. The plugs were toast. I mean rusted, worn, almost rotten. The wires had to be cut off. Original plugs and wires. My Honda calls for a first plug replacement at 120.000 and when I pulled them they looked new (Denso.) These Autolite plugs probably should have been replaced at no later than 40,000 miles. I don't know what the service interval actually is, I just know that it's a MIRACLE this car went 120,000 miles on plugs that were trash. SO, it runs a bit better now, and I think it's the best it's going to get. I had no idea it was only 193 hp. What a waste to put that engine in such a beautiful car. Oh, and the check engine light is still on. Nobody know why. I've read this is common. If anybody is actually on this forum and could confirm I would appreciate it.