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When to Replace Spark Plugs
Replace spark plugs on the interval in your manual for the plug type the engine uses. Copper and iridium are not on the same schedule.
When to replace spark plugs is written in the maintenance schedule for your engine, and it changes with the plug. A traditional copper plug is often due far sooner than an iridium or platinum plug the factory fitted. Copying a '100,000 km' figure onto a car that left the line with copper plugs, or changing iridium plugs every oil service, are both ways to get the job wrong.
The manual wins. So does the plug number it specifies. A hotter or colder plug than the book, or the wrong reach, is not an upgrade you should invent. If a shop fits a different number, the invoice should say why.
A misfire, a rough idle, or a flashing check-engine light is a reason to have the engine diagnosed. Plugs, coils, compression, and fuel can all do that. Replacing plugs because a lamp came on, without reading the fault, spends money and writes a misleading row. If the light is flashing, that usually means stop driving and get a diagnosis, not finish the trip.
After a real plug change, log the date, the kilometers, how many plugs, and the next interval from the book. RPMBase stores that confirmation. It does not read misfire counters and it does not know which plug your head takes.
The interval depends on the plug
Read the maintenance chapter, not the packaging of a plug that happened to be on sale. Factory long-life plugs are a reason the gap is long. If a previous owner fitted short-life plugs, the long factory interval no longer applies until the specified plug is back in the engine. The invoice from that visit is how you know.
Severe use, if your book has that column, can shorten the gap. Short trips, heavy load, and fuels the manual warns about belong there. Do not invent a shorter gap for drama, and do not ignore a shorter gap the book already printed.
Engines with more than one plug per cylinder need every plug the procedure names. A set means the set. Logging 'plugs done' when one cylinder was left for later is how a misfire survives the receipt.
What a plug change is, and what it is not
The job is removing the old plugs, checking they are the right type and that the gaps are what the spec wants if the procedure says to gap them, and fitting the new ones at the torque or method the manual describes. Some plugs are not meant to be gapped by hand. The shop should follow that.
Ignition coils and plug wires are not the same part. A coil that failed is a coil. A plug change does not automatically include coils. If the invoice lists coils, log them. If it does not, do not add them in the note because the engine runs better.
A throttle cleaning, an injector service, or a 'tune-up' package can hide whether the plugs were actually replaced. Read the lines. How to read a service invoice is the habit that keeps a package name from becoming a false history.
Symptoms that deserve a diagnosis first
Rough running, poor cold starts, and higher fuel use can be plugs. They can also be air leaks, sensors, or fuel. A stored fault code is the workshop's starting point. Your log can say 'diagnosed, plugs replaced' only if both of those things happened.
Oil on a plug tip, or a plug that looks very different from its neighbors, is information for the person doing the job. It can mean the cylinder has another problem. Firing the parts cannon and logging four new plugs does not close that question. Write what the shop found if they told you.
Do not clear a warning light and call that maintenance. If the light returns, you do not have a spark-plug interval. You have a fault. The maintenance tracker should not be where warning lights go to disappear.
What to record so the next interval is real
Date, odometer, plug specification if it is printed, quantity, and cost. Next due is the book's interval added to this odometer, or the date if the book also gives months. If you do not know the interval, look it up before you type a round number.
Keep the old plugs only if you need them to show a shop. Otherwise the invoice is the proof. A photo of the box, attached to the row, beats a memory of the brand.
Set the reminder on that row. Plugs are easy to forget because the engine still starts. The routine schedule is full of jobs like that. The reminder is how this one survives until the kilometer arrives.
How plugs relate to oil, filters, and fuel
An oil change does not include plugs unless the invoice has a plug line. They share a visit only when someone scheduled them together. Split the rows if the next due points differ, which they almost always do. The oil interval is its own decision in how to choose an oil change interval.
A clogged air filter can make an engine feel weak in a way people blame on plugs. The filter is a different part, usually on a shorter interval. Keep filters on their own line so you do not replace plugs to fix a dirty filter.
Fuel economy that suddenly drops is a reason to look at maintenance you skipped, including plugs, filters, and tires. It is not, by itself, an instruction to buy parts. The fuel tracker shows whether the drop is real over several fill-ups or one expensive tank.
Used cars and missing plug history
If you bought the car with no record, you do not know the plugs. Looking up the interval and either inspecting or replacing them starts a clock you can trust. Guessing that 'they were probably done' is the same hole as a missing service history on a used car.
Write unknown for the previous set. Write a real row for the set you install. Future you, and a buyer, can tell those apart. Service records when selling are easier when this job has a date.
RPMBase will not backfill a dealer network. You type the visit or you confirm a draft from the invoice photo. A blank is allowed. A fictional 100,000 km service is not.