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My girlfriend has a 2007 Nissan Sentra with 160000 miles on it. She does not think the spark plugs have ever been replaced which leads me to believe that the ignition coils have also never been replaced. She does not have any check engine light codes for any misfires or any other problems.

I am planning on changing her spark plugs next week and I'm wondering is it worth the cost to preemptively replace the ignition coils as well? Or since I do not have any check engine light codes should I just leave the ignition coils alone?

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    Changing the plugs is just fine. Coils are not maintenance. They are replaced as they fail. You might find it more beneficial to find the maintenance schedule for this vehicle and catch up on the routine maintenance that has also been neglected such as transmission fluid change, brake fluid change, coolant system flush and timing belt replacement (if it has one). – Jupiter Feb 3 at 18:31
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    @Jupiter this is an answer – Solar Mike Feb 3 at 18:59
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    I copied this to an answer. I'm fairly new to this site and have not quite gotten a grasp on the logistics. Hope you'll be patient with me. I like the knowledgeable responses that are supplied to people's problems. – Jupiter Feb 4 at 18:35
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Effectively speaking, some automotive parts slowly wear or fail in a way that is predictable. For instance, we know that oil filters collect debris over time, so it's easy to predict when you will need to replace those parts based on how old they are or how many miles you have driven. For these parts, even if they seem to be working okay right now, you can plan on replacing them based on their age or service life.

On the other hand, some components don't fail in a way that lets us predict their useful service life based simply on their age. In other words, we don't typically expect to replace them after a certain number of miles, and it's generally best practice to wait until they've actually failed (meaning: they are no longer operating acceptably, regardless of whether that is a literal complete failure or just a partial failure that leads to bad performance) before replacing them.

From a statistics perspective, this concept is called a memoryless distribution - age doesn't play into the rate at which these parts die. If the part is working fine right now, the chances of it dying today are the same no matter how old it is.

Think of your ignition coils the same way you think of light bulbs in your home. If you had a light bulb that was 5 years old, and working perfectly fine, would you go and replace it right now, just because it's five years old? Probably not, right? You would probably wait until it dies (or at least shows signs that it's dying), and then put a new light bulb in it's place. That brings us back to your actual question: Should you replace a perfectly-functional coil, just because it's old? And the answer is, no: if the coil is not showing any signs of failure (you have no drivability problems, you have no check engine lights, etc), you should feel perfectly comfortable not bothering to replace it right now.

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    wrong, ign coils cane become weak and not supply specified voltage or milliamps across the spark plug gap. – Moab Feb 4 at 1:26
  • @Moab ...which would be a failure (they don't work any more.) My point was, it's not like a brake pad which wears out predictably, in a way that lets you say "at this much wear, or after this many miles, we expect to replace it." – dwizum Feb 4 at 13:49
  • I don't understand what you think is wrong and I think you're misinterpreting what I'm trying to say. If it's weak to the point that it's causing emissions issues, wouldn't that count as a failure? Also, can you plot a curve that shows how they fail over time? (I can, after having worked doing failure analytics for one of the major suppliers of ignition coils, which is what I based this answer on). – dwizum Feb 4 at 14:01
  • Again, my point is - they don't slowly wear out in a way that is predictable, which means there's no point in replacing them just because they're X years or Y miles old. Instead, they just eventually stop working in an acceptable manner, but you can't use the current age to predict when that will happen. So, you should not try to pre-emptively replace them, you should wait until they fail (including weak performance or a literal complete failure) and then replace them. – dwizum Feb 4 at 14:02
  • "coils either work or they don't work." this is what is complete nonsense. – Moab Feb 4 at 14:04
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Changing the plugs is just fine. Coils are not maintenance. They are replaced as they fail. You might find it more beneficial to find the maintenance schedule for this vehicle and catch up on the routine maintenance that has also been neglected such as transmission fluid change, brake fluid change, coolant system flush and timing belt replacement (if it has one).

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  • Glad you posted your answer. – Solar Mike Feb 4 at 18:46

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