There's that many factors in your question that could contribute to any number of problems. Under normal circumstances, a correct spark plug shouldn't cause any engine problems. If anything, it would probably be the other way around. I've been around long enough to know that if you have problems with incidentals, [especially if you put them in yourself], a dealer will tell you to take it up with the component company. Fouled plugs can be caused by a variety of issues, but assuming your engine is ok, it would be highly unlikely for a brand new plug to be defective. I know, it happens, but I doubt it. It's pointless replacing the plugs or the oil if the head gasket is blown. Is the head gasket blown and if it is, where is it blown? If you have water in your cylinders, the plugs would also show signs of rust. Get it compression tested then check for a valve problem. Dirty valves do not seat properly and that would be a major cause of plugs fouling up.

"A family friends shop"? Pretty lame mechanic if he can't even do a compression test to see if there is a dead cylinder!!! I replace fouled spark plugs and your friend probably put the wrong brand in if the dealer doesn't like them. A dead cylinder will cause the gas that isn't burnt to wash down the cylinder wall and dilute the oil. Warranty repairs the mechanical problem. You decide if you want to be smart and take care of the oil change and spark plugs.

Tell him you will do the oil change, spark plugs, air filter, and anything else that is not warranty, later.

None of these things could possibly effect the low compression.

And in particular, one does NOT change fouled spark plug, but simply cleans them.

That has been true for about a century.

Every shop has a spark plug cleaning blaster because it happens so often.

Explain in more detail please.

My 2011 dodge avenger with 52,000 miles on it recently broke down. I have a extended power train warranty on the car. I took it to a family friends shop said it was reading no ignition/low compression on cylinder 6. He replaced the spark plugs and still no change. So he determined it had to be an engine issue and I should take it to the dealer as it will be under warranty. So I had it towed over to a local dealership. The dealer first called and said it will be under warranty, but I need an oil change air filter and new spark plugs. I asked him why on the spark plugs and informed him that are just replaced. He claimed they were the wrong time. I emailed the receipt of the ones the mechanic put in along with the mechanics number and he then claimed that the we're the correct ones. But they were fouled and needed to be replaced. I then told him that the car hasn't been driven since the plugs were changed, it was towed to his dealership right after they were installed. He then said he had really only looked at two of the plugs. But because I didn't buy the plugs from them the can't warranty the plugs. If there is a blown head gasket or something else, that is the reason the plugs were fouled in the first place. Shouldn't they be replaced? It's not a defect with the spark plugs themselves, just damage from the issue that falls under my warranty. I'm really not liking these people. I also think if they are breaking into my engine that the oil change should also be included.