I have a 2018 Mazda 3 2.5 with 103k miles, i bought it with 33k miles.
I drive roughly 30k miles a year and i do an oil change every 5k so I'm doing the basics.
At around 50k i was told the tensioner was leaking so i replaced it, they didn't say anything about the belt so im assuming It's the original one.
Should i replace both of them now as part of maintenance or wait until i notice symptoms?
Or should I only replace the belt since it's got about a 100k on it?

If you really want to push off maintenance... Keep an eye on the wear indicator.

In reality... The belts are cheap and easy to do from the top with an offset wrench. So why not replace it for $20?

 

I'm not really handy when it comes to things like that so basically the only way is to get it serviced somewhere.
The dealership quoted me about $600 for tensioner and belt replacement a few months ago.
By looking at the belt it looks ok, i didn't see any cracks on it. The tensioner however has black dirt/grease on it so it might be leaking?

 

Start shopping around for another quote. If the shop is good, it should only take them an hour from the top and cheaper than dealership. The backside of the belt is not the issue unless there are obvious signs that it's splitting into the material. It's the grooved side that needs to be flipped around and bent back to check for splitting, unless it's actually obvious that material is splitting.

Grab some brake cleaner or soapy water and spray off all dirt and clean up what you can. Inspect again next week if it's leaking. If it is leaking, start shopping around. Mind as well get the belt replaced if it's coming off if you suspect the tensioner is going bad.

 

Thanks! I'll try cleaning it and I'll check other shops

 

I wouldn't use brake cleaner around there, or at least I'd do so very carefully. Brake cleaner probably isn't a good thing for the belt. use some sort of degreaser on a cloth and wipe things down by hand.

I would recommend replacing belts too. If they break, it'll take out your water pump and then your motor overheats. Replacing two belts is a lot cheaper than replacing a motor.

If you can handle an oil change replacing belts isn't much more difficult, same goes for the tensioner. Just need to do lots of research first is all.

There are some very good non OEM options for parts if you want to save some money. For example, gates makes most of the belts and tensioners for OEMs. In other words you buy a belt from a dealer and it'll have gates stamped on it...for most cars anyways. You can buy gates parts elsewhere for cheaper and it's the same thing.

 

Best tensioner bar none. https://www.amazon.com/gp/product/B09GS3FGWB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Just go to autozone or whatever and pickup belts(get the alternator while there).

I usually use this search for automotive service centers.

https://www.amsoil.com/locator/map.aspx

Shops that use amsoil have always been among the best and honest shops in my experience and the labor to replace belts(replace the alternator one while there) and tensioner shouldn't be more than 200-300 bucks.