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I have a 2014 Sienna approaching 120K miles; it's recommended to change out the spark plugs now. Is this just a precaution, or is it necessary? Can I expect improved mileage/performance with new plugs? Thanks!
 

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Spark plug gap grows as the electrode wears. Larger than spec gap puts extra strain on the coil packs, reducing their life expectancy. As gap grows, plugs become less efficient and can lead to loss of power, reduced fuel economy, and other driveability issues.

While they're back there, I'd at least do the rear upstream O2 sensor. Upstream O2 sensors are a normal maintenance item in Europe as they get lazy affect fuel economy.
 

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Are you going to do it yourself or take it to a shop? diy, I took 4 hrs or so. Saved over $700 compared to having it done at the dealership. I also cleaned the throttle body. I'm sure the dealership would charge another lovely $200 for that.

The question is, why wouldn't you do it? Besides what kind of gas mileage are you expecting. Are you not getting 18/24? I am. Before and after the change.
 

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I had the plugs replaced on my '12 at 135K miles. Surprisingly there was virtually no wear or degradation of the electrodes.
However, it is advisable to replace plugs at 120K miles for reasons given above.

While not necessary, I also had the mechanic replace the rear COP coils at the same time (no additional labor.) The coils were $90 each (Denso.)
 

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Manufacturer maintenance recommendations have a factor of safety so 120K is not a hard #. if the van is not displaying any startup or driveability issues then their is no imminent need to swap the plugs. Bad fuel or carbon buildup is usually what shortens plug life but if that was the case you would have known.

You should be fine pushing out the plug swap to another 5/10K or even more but If you are OCD about maintenance then go ahead, do it now. Meanwhile check out the forum DIY posts on plug swap, some with pics of old/new plugs.
 

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I have a 2014 Sienna approaching 120K miles; it's recommended to change out the spark plugs now. Is this just a precaution, or is it necessary? Can I expect improved mileage/performance with new plugs? Thanks!
As a wear item you are not wasting money by doing them at that mileage for all the reasons other people's have stated above.
Plus it's cheap insurance to do them now at YOUR convenience to prevent a inconvenient mandatory repair later.
 
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