Q: Three coils are misfiring. Do I have to change all three at the same time? Can I change the spark plugs first?

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I am looking for the most cost efficient way to take care of this.

My car has 124000 miles.
My car has an automatic transmission.

Ignition coils deliver the spark to the spark plugs. If you are confident that the coils are in fact misfiring, changing the spark plugs will not fix your problem. Ignition coils are coated with a varnish-like insulation that becomes brittle over time after being stretched then contracted repeatedly The insulation develops small fractures that open when heated and close when cooled. When open, they allow shorting of coil windings and decrease or eliminate the coil’s ability to function. This type of temperature cycling happens because every time you shut the engine off, the fan stops running, the coolant stops flowing and the temperature under the hood rises. Every time you start the engine again the coolant flows, the fans turn on, and the temperature drops. When this heat dissipates and is absorbed under the hood, it is trapped causing this cycling effect. I would recommend having an expert from YourMechanic come to your location to take a look at your vehicle to properly diagnose your ignition system.

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