I am embarassed to say that my 2007 Mazda3 (2.5 L) has never had the spark plugs changed (165,000 miles). Any special precautions that I need to know to replace them? (I have no misfires.)
Then don’t mess with them.
My Accord has 220,000 miles and the plugs have never been replaced.
Runs great!
Tester
Never buy a car from a mechanic I have heard. The biggest downside I can see is some plugs get so locked in place removing them can be a major issue. (Love all @tester does to help other people, no offence intended.) I prefer to stick by recommended maintenance schedule.
Do it with the engine stone cold.
Don’t try to force them out in one move.
If they seem to hang up, put on a little penetrating oil and work them back and forth.
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I’d inspect them at least. I’d also replace them and look for any difference in compression and fuel mileage; if you get an emissions test, see if they make a difference.
Use the exact plugs spec’d by the carmaker.
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That makes no sense. If you’re going to take the time to remove them, then just replace them. I see no value in removing them to just inspect.
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Mazda would typically spec NGK or Denso plugs as OEM. Both use a special coating on the threads (NGK uses a trivalent plating and Denso is similar) so they should come out of the engine with relative ease, even after all these years.
I second the vote to use the exact same plugs. These have been perfect, what more could you want? I’d expect that you might find that the flexing and pulling on the spark plug connectors might cause some issues, so be ready to replace them as well.
Or do what @Tester does, which is nothing. Watchful waiting.
Like the OP, I would be curious and want to take a look. If they are OK, they probably could keep on working, like they do for Tester. If I wanted to keep using them, I would gap them.
No way would I go to the effort of pulling the plugs and not replace them with new. What, to save $30-40?
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I asked the question, so here is what I plan: if it ain’t broke, don’t fix it. I going with Tester!
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Old plugs with overgrown gaps might not misfire, but they will put more stress on ignition coils, leading to their earlier demise.
It’s funny to me how some on this forum will flip out if someone suggests running oil a little longer or one step thicker than the Owner’s manual suggests, but others have no problem running spark plugs 50% longer or more just because there’s no immediate symptoms.
(How’s that for a run-on sentence!)
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Customer: How much to replace my spark plugs?
Technician: $500.
Customer: Why?
Technician: In order to replace the back spark plugs, requires I remove the intake system of the engine.
Customer: What do you suggest?
Technician: Drive the vehicle until the Check Engine light comes on with a misfire code, then I’ll have to remove the intake system of the engine.
Tester
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OP is not spending $500 to change plugs on a 4 cyl Mazda.
Alert: The Wizard of Oz wants his straw man back. 
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The plugs can be okay but something wrong with the fuel mix or coil, etc, would show up in the plugs. It’s possible to learn something from inspecting a plug that’s still good.
It certainly depends on how hard it is. I read about a Monza that required lifting the engine to get to the plugs. It’s a piece of cake on my '87 Toyota pickup. I inspect the plugs annually.
About 10 years ago 1 rattled loose; fortunately I was on a dirt road in a national forest so could stop where I was, take as long as I liked dealing with it. Inspecting them annually keeps them from getting stuck and makes me tighten them.
The maintenance schedule for my 2009 Outback calls for spark plug replacement at 30K miles. Same for my 2005 4Runner. As near as I can tell the maintenance schedule for a 2007 Mazda 3 calls for new spark plugs at 37,500. This is according to the owner’s manuals. Are we now being told to ignore the owner’s manual, which otherwise is the standard recommended reference that everyone directs posters to on the forum? How did this basic reference suddenly become irrelevant?
I replace spark plugs on customers vehicles on schedule because that is business, if you tell your customers to defer the maintenance indefinitely they will find a different shop to maintain the vehicle.
Typical replacement of iridium spark plugs is at 120,000 miles.
Last summer a customer with 248,000 miles on the vehicle came in with a check engine light, I wrote an estimate for the repair and the service writer asked me to “check the spark plugs” per the customer’s request. OK, 3.4 hours to remove the spark plugs and for what reason?
The customer said that they have never been replaced. I suggested that he have the spark plugs replaced after returning to California, replacing the spark plugs on this vehicle involves disconnecting a lot of hoses and wiring connectors and I didn’t want his trip delayed waiting for replacement part should something break. He asked to have the spark plugs replaced so I spent 3 and a half hours stretched across the engine replacing spark plugs.
The gap on iridium spark plugs changes very little, at 120,000 miles the tips are still square.
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Given the change interval for most new plugs are at 120K miles, the plugs might not be stuck after all.
My Veloster turbo calls for plugs every 45K miles, same engine without Turbo goes for 120K miles on the same plugs.
The OP seems to have decided to keep the plugs and drive, just curious if OP is the first owner of the car or purchased used?
Yes, I am the original owner, so you can imagine a few other schedule items I have skipped. Oil and air filter have been on time. I have never needed brakes.
I thought that spark plugs were part of the emission system that was required to be designed for 100,000 miles.