Spark plugs have a real interval — and a genuine inspection shortcut the parts menu never mentions.

What the schedule actually says

Copper/nickel plugs: 30k. Platinum: 60–100k. Iridium (most cars since roughly 2005): 90–120k, and several manuals say inspect at 100k rather than replace. Direct-injection and turbocharged engines burn plugs faster than the schedule assumes — that is the legitimate case for the lower half of the range.

The inspection that beats the calendar

Pull one plug (ten minutes on most four-cylinders) and read it: a normal gray-tan insulator with a rounded electrode means the set has done nothing wrong. A white or vitrified tip means lean or detonation problems coming. Oil-fouled means valve-cover gaskets or rings. The plug is a diagnostic sample — throwaway replacements throw away the data.

How this job gets oversold

The tune-up bundle — plugs plus wires plus filter plus throttle-body cleaning plus fuel-induction service because it is all on one page of the menu. A misfire code gets a set of premium iridium plugs sold on a car whose actual fault was a $20 coil that logs the same code all by itself.

The measurement that settles it

The real test is watching the coil fire with a scope, reading plug condition on the bench after pulling, and a misfire-counter test-swap of ONE coil before buying a set of anything. Resistance race-testing a modern iridium plug tells you nothing.

Straight answers

Do I really need to replace all the spark plugs at once?

Usually yes — a fresh plug carries less ignition load than its worn neighbors, and the mix makes the oldest one's failure look like a new plug's fault. But all-at-once describes the set, not the price tier: a manual-correct nickel or iridium plug at half the retail of the premium brand is the right answer for the interval the car was engineered around.

My plugs were replaced at 60k but the car misfires anyway. What now?

Then the plugs probably are not the fault — and a shop that sold you a second set without testing ignition signals on the first complaint sold parts, not a diagnosis. Misfire codes follow the coil when you swap one coil across cylinders; that single two-minute test tells you whether to buy anything else from the ignition aisle or go look at fuel delivery, compression, or a vacuum leak instead.