Fixed counter issues on my 6-year-old PA-II. Easy to install but having written instructions (as alternative to a video) would be useful. Would buy again if needed.

Dependable source of PMS5003 counters vs. hit or miss AliExpress adventures

Yes, you can buy a supply of these sensors directly from China, but PurpleAir tests the lots of sensors they ship out so you have a good chance of problem free counter replacement at maybe 10% of the original cost of the PA-II+ sensor per year. I only have one PA-II+ sensor, so I'm OK keeping spare counters ready to drop in when counts go wacky and cleaning doesn't help. On paper the newer dual laser based counters for the newer PurpleAir sensors should require 1/4 or fewer repairs, but annual repairs are OK. At first I thought the "flat shipping" was pricy, but 2nd day FedEX was a pleasant surprise.

laser counters work great but adapter kit hard to install

i needed to replace the sensors on an old PAII. purpleair support recommended the adapter kit. i found the kit hard to install. the YT video was difficult to follow because of constantly shifting point of view. also, i had to exert a lot of force to get everything to fit into the old PAII shell. the adapter kit is a good idea but it could have been made a lot more intuitive, as well as avoiding the fit issue. there were no text or printed instructions. i think if they had tried to create printed instructions, they would have seen the need to simplify the process.