Again, put in what was in there in the first place. Whether they are 60K or 100K plugs makes no difference. Leaving plugs in for 100K is asking for trouble. Change them out at 60K and go on with your life. Prevention is a lot easier than extrication.
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Are you able to take out one of the front plugs and see what is in there right now?
It sounds like you havenât tried that, and if you already have the Denso plugs in there, doesnât that answer your question?
Aside from the brand/type of plug, you can also look at the plug and compare it to various âwearâ charts/pictures on the internet to decide how âwornâ they are.
The OEM Denso SK20R11 spark plugs have a service life of 120,000 miles in your Sienna, disregard any other advertised claims, not all vehicles are the same.
You may find NGK spark plugs in your engine, Toyota used either Denso or NGK during assembly.
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My 05 4runner came from the factory with Denso on the Drivers side and NGK on the Passenger side. Many other 4runner owners found the same thing.
That screams interchangeability. I think most of us would agree to that since this is Toyota. I wonder if that would be the case if FCA mixed plugs, even if they are interchangeable.
Itâs the edges. Arcs form first from sharp points and edges. That flat faced electrode? If you captured images of the arcs, they would be dancing around from the sharp edge of the center electrode to the ground strap. Look at a worn plug. The center is a dull point with the outer edges worn away. Which brings up another aspect. Material is being transferred from one side to the other. Why do you think that center electrode is a special alloy and the ground strap is just plain old steel? Hint itâs not just thermalâŠ
Makes sense. Just like lightening tends to strike the point of a mountain, or the top of a pine tree or tower.
Iridium is harder than platinum, and in most cases, iridium spark plugs last about 25 percent longer than comparable platinum spark plugs. Because iridium is costly, iridium spark plug manufacturers reduce the diameter of the center electrode to as little as .4mm. In addition to saving money, the âfine wireâ center electrode on iridium spark plugs increases firing efficiency.
Many carmakers require iridium spark plugs or iridium/platinum combination spark plugs for (COP) ignition systems. If your ownerâs manual specifies iridium spark plugs, donât downgrade to platinum spark plugs, or double platinum spark plugs, or even copper spark plugs. They wonât perform as well as the iridium spark plugs in this case.
As the spark ignites the air/fuel mixture, the colder side electrode tends to âquenchâ the flame. To combat quenching, some spark plug manufacturers cut a âUâ or âVâ shaped channel into the âspark receivingâ surface of the side electrode. The larger channeled area reduces quenching and allows the flame to grow more quickly. Other manufacturers split the end of the side electrode to reduce the flameâs contact with the side electrode and allow the flame to shoot straight down into the cylinder.
https://shop.advanceautoparts.com/r/advice/car-maintenance/differences-between-spark-plugs
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Denso and NGK are OEM sparkplug providers to Toyota. Some Toyotaâs came with nothing but NGK plugs, others came with Denso and others came from the factory with a mix of NGK and Denso. When I them I bought all NGK plugsâŠdidnât buy 3 NGK and 3 DensoâŠ
I know what you meant. My comment concerned the manufacturer mixing plugs, not you.
Yes I knowâŠI was being facetious.
OK. You had your hand over your mouth and I couldnât tell you were smiling. 
124k is a long time for any plug, though I imagine the iridium style can take it. (The iridium plugs in our 06 Civic looked good at 110k.) Over the years several friends have had trouble when leaving plugs in too long - over time the plugs can gall and strip out the threads in aluminum heads (only remove them when cold) and once an old insulator fractured scoring up a cylinder wall.
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