Once a year, ideally, at the start of the BBQ season, you'll get your pellet grill/smoker out for a deep clean.

First off, you'll want to empty the hopper of any pellets and get a vacuum/shop vac in there to suck out any pellet dust/fines that are sitting in the base of the hopper. This dust doesn't burn efficiently.

Therefore it can cause issues with temperature regulation. If there were pellets still in the hopper, if they pass the snap test and they break cleanly/still have a surface shine, you can load them back into the hopper.

After taking out all the cooking racks/grates to get down into the belly of the pellet grill/smoker check over the burn pot. If its heavily corroded/rusty, you might want to consider replacing it. Also, take this opportunity to check the hot rod igniter is functional, as if its broken this will be the best time to replace it.

When it comes to cleaning the cooking chamber and cooking racks, warm water and a suitable degreasing agent will work. Wire wool can be used but be careful not to damage any painted finishes/or enamel coatings. 

You'll also want to carefully clean the RTD probe in the cooking chamber. This is the probe that tells the pellet grills controller what the actual temperature of the cooking chamber is. However, its just a thin wire inside a metal jacket, and it can snap if damaged. A failed RTD probe is one of the most common issues that can cause a pellet grill/smoker to stop functioning properly.