Lift the hood wherever the car is.

Run your scan. This takes about 30 seconds of reading time if you only have to scan the check engine light rather than scan all systems. So a couple minutes max including going to get the device.

It should tell you which cylinder is misfiring. You don’t have to replace *all* of the plugs and coils when you have a misfire in one cylinder. Sometimes it reads as multiple and then will have a code for a single cylinder as well. Your mechanic knows that 90% of the time, replacing that one plug and coil will fix the problem, but they might try to get you to do all 6 anyway.

For a mechanic to diagnose and replace one spark plug (which he already has on hand, unless it’s a crappy shop) takes 20 minutes or less. Realistically, significantly less.

Find and use 4 total tools:
1) OBDII reader
2) Ratchet
3) One small socket to remove the nut that holds the coil on.
4) One other angle socket to remove the plug.

None of these steps individually takes more than a minute:

- Unscrew coil nut

- unplug and remove coil

- unscrew plug

- insert and ratchet down new plug.

- pop in coil and snap on leads (it’s literally that easy.)

- tighten coil nut

That’s it.

Run your scan again to confirm.

If you’re already switching the plug, putting in a new coil too takes literally no extra time (except maybe 5 seconds to open the box?) and no additional tools, so if you’re charging your customer for it anyway, it’s not taking you any more time.

Done.

I literally did this last month. You’re right that driving to the parts store might take most of the time, so I now have a plug for each vehicle in the glove box.

I’m not a pro, but since I know where my tools are and how it’s done, this takes less than 10 minutes unless I want to spend more time confirming diagnosis with multiple position changes and multiple reads, etc. to save the cost of one part or another. A mechanic who replaces all 6 of both isn’t trying to save you anything. He could replace both plug and coil in that 10 minutes.