I took my 2004 Mazda to maintenance but the mechanics told me the mazda has iridium spark plug on it and doesn't need to be replaced until 120k miles. They even pulled one out and checked and said it should be fine.

Is that something for you guys as well or I should replace them anyway at 75k?

 

Four comments:

1. The Owner's Manual for my 2009 Mazda3 recommends Spark Plug replacement at 120,000 kilometers or 75,000 miles, that said, it's certainly possible that the mileage recommendation has changed since your car was built.
2. Several of our previous cars have recommended the plugs be replaced at the 100,000 mile mark. After eight years I replaced the plugs on the first one at almost exactly that point and saw that they were in pretty good shape, so when our second one hit that point I decided to wait until the following spring. When I pulled those plugs out at 120,000 I wasn't terribly surprised to see that they were also in pretty good shape.
3. After each of the two above plug changes, drivability and fuel economy didn't change to any measurable degree.
4. For my Mazda3 I'll probably target the plug replacement for somewhere between 75,000 and 100,000 miles.

 

My '04 manual says the same, 75k miles for plugs, not 120.

 

mine says the same as well, BUT the pos bosch plugs that were in mine went bad after 16k, ive had the same autolites in them ever since (i have 90k+) and there still fine.

Other stories ive heard people have only had luck with NGK's, my suggestion is to leave them as is until something happens.

 

Replaced plugs at 105,000 miles; old plugs looked great and car was running perfect; put in NGK Laser Iridium per OEM spec and car runs fine but seems to have less power and pickup, and mileage is down slightly. Any thoughts why?

 

If you're curious then pull the plugs and see how they look. If they look worn then replace them. I do recommend going with the OEM NGK plugs..... they're only about $13/ea so not terrible.

 

A good thing to do is to pull the plugs every 20k even if they are 100k plugs, doing this at the very least keeps them from seizing in the head and/or stripping out the threads, plus you can keep an eye on how the engine is running.

 

$13/a piece. Boy how times have changed. I remember when Champions were 0.80 ea and others were still under $1. Of course they weren't 100k mile plugs then either. But hey, I could change my plugs 16x more often for the same $$$.

I changed my OE plugs @ 70k miles, and they too looked fine.