Fi’s naming has confused a lot of shoppers. The Series 3 launched in October 2022 and got reviewed everywhere; the Series 3+ arrived in mid-2025 and looks identical in photos. If you’re staring at two listings that share a shape, a size chart, and most of a name, here’s what separates them — and what’s coming next.

Quick Answer

Buy the Fi Series 3+ — it’s the current model and the one Fi sells today. The ”+” adds AI behavior detection (scratching, licking, barking, eating, drinking), roughly double the GPS sensitivity, app-customizable LED light colors, Apple Watch support, and a reinforced stainless-steel frame. Only go looking for an original Series 3 if you find leftover stock at a real discount and you genuinely only want location tracking. And no — there is no Fi Series 4 as of August 2026. The Series 3+ is the newest generation.

Specs Side by Side

Feature	Fi Series 3 (2022)	Fi Series 3+ (current)
AI behavior detection	No	Yes — scratching, licking, barking, eating, drinking
GPS sensitivity	Baseline	About twice as sensitive, per Fi
Collar light	LED light, fixed	LED light, color selectable in the app
Apple Watch app	No	Yes
Housing	Stainless steel casing, 33% stronger than Series 2	Reinforced stainless steel frame
Physical shape and size	Identical between generations	Identical between generations
Band compatibility	Shared across Series 3 and 3+	Shared across Series 3 and 3+
Water resistance	IP68	IP68
Network	Cellular + GPS	LTE-M cellular + GPS
Activity and sleep tracking	Yes	Yes
Escape / lost dog alerts	Yes	Yes
Battery	4–5 weeks typical, up to 3 months light use	Up to 3 months in activity-tracking mode
Subscription	Required for tracking	Required for tracking
Sold new today	No — older stock only	Yes

Read that table and one thing jumps out: the physical device barely changed. Fi didn’t redesign the collar for the 3+. It kept the shape, the size range, and the band system, then upgraded what’s inside and what the app does with it.

What the ”+” Actually Adds

AI behavior detection is the headline

This is the real difference, and it’s a different category of feature from anything the Series 3 does. The Series 3 tracks where your dog is and how much they move — steps, activity goals, sleep. Fi’s own announcement of the Series 3+ frames the addition as trained AI that identifies specific behaviors from the collar’s motion data: scratching, licking, barking, eating, and drinking.

Why that matters is less about novelty and more about veterinary timing. Persistent scratching or licking is the earliest visible sign of allergies, skin infections, and pain — the Merck Veterinary Manual’s overview of behavioral problems in dogs is explicit that repetitive licking and chewing frequently trace back to a medical cause rather than a behavioral one. A change in eating or drinking frequency is exactly what your vet asks about first and exactly what you can never answer precisely. A collar that logs those events while you’re at work turns “he’s been acting weird lately” into a dated pattern you can show someone. If health monitoring is the reason you’re shopping at all, this is the feature to weigh against everything else.

GPS sensitivity roughly doubled

Fi states the 3+ is about twice as sensitive on GPS, which shows up as faster fixes and tighter accuracy — not as a new number on the box. The Series 3 was already competent here. The 3+ shaves time off the moment that matters, which is the first thirty seconds after your dog goes over the fence. For dogs that actually escape, that gap is the whole product, and we cover the category on that basis in our guide to GPS trackers for dogs who escape.

Two small things that are nicer than they sound

The LED light on the 3+ lets you pick the color in the app. That’s cosmetic until you’re walking two dogs at dusk and want to tell them apart at fifty yards. And the Apple Watch app means checking your dog’s location without pulling out a phone — genuinely useful on a run or a hike, mildly pointless otherwise.

Who Should Skip the Upgrade

If you already own a working Series 3, the case for replacing it is narrower than the feature list suggests. You already have live GPS, escape alerts, activity and sleep tracking, the IP68 and IP66K durability Fi described at the Series 3 launch alongside a stainless frame rated for over 400 lbs of pull force, and the same physical collar. What you’d be buying is behavior detection and a faster GPS fix.

Upgrade if: your dog has a chronic skin or allergy issue you’re trying to document, or your dog is a genuine escape artist where seconds matter.

Stay put if: you bought the collar to answer “where is my dog,” it does that reliably, and your subscription is current. Fi has said that when a new generation launches, subscribers in good standing become eligible to upgrade to the latest hardware — so keeping an active plan is itself a hedge against being stranded on old gear.

Buying an Older Series 3 Now

You can still find original Series 3 units in leftover retail stock and on the secondhand market. Two cautions before you chase the discount.

First, a used Fi collar has to be transferred out of the previous owner’s account before it will work on yours. Buy from someone who can confirm they’ve released it, or you’ve bought a paperweight. Second, the collar is only half the product — GPS tracking requires an active Fi subscription regardless of which generation you own, so a cheap collar doesn’t get you a cheap tracker. If ongoing fees are the sticking point, look at GPS dog collars with no monthly fee instead of hunting for old Fi hardware.

The good news on accessories: bands are shared across Series 3 and 3+. Third-party Fi collar bands fit either generation, so you’re not locked into Fi’s own straps and you won’t strand a band collection by upgrading.

FAQ

Is there a Fi Series 4 coming in 2026? There’s no Fi Series 4 on the market as of August 2026. The Series 3+ — launched in mid-2025 — is the current generation and the version Fi sells today. Fi does reference future-generation upgrade eligibility for active subscribers, which implies a Series 4 will eventually exist, but no release has been announced.

Is the Series 3+ worth it over the Series 3? If you’re buying new, yes — the 3+ is what’s actually for sale, and the AI behavior detection plus the improved GPS sensitivity are meaningful additions rather than spec-sheet padding. If you already own a Series 3 that works, the upgrade is optional unless you specifically want behavior data.

Do Fi Series 3 collar bands fit the Series 3+? Yes. The two generations share the same physical module shape and band system, which is why third-party makers sell bands listed as compatible with both. It’s one of the few genuinely consumer-friendly decisions in the smart collar market.

Does the Fi Series 3+ still require a subscription? Yes. GPS tracking and the connected features run over an LTE-M cellular connection, and that requires an active Fi membership. This is standard across cellular pet trackers — a tracker that reaches your phone from anywhere needs its own data plan. Tag-based alternatives avoid the fee but only work within Bluetooth range or a crowd-finding network, which we break down in our Fi collar vs Apple AirTag comparison.

How long does the Fi Series 3+ battery actually last? Fi advertises up to three months in activity-tracking mode. That figure assumes the collar mostly stays inside your home’s known Wi-Fi zone and rarely wakes the GPS. With regular GPS use, expect substantially less. And in Lost Mode, which switches to continuous live tracking, the collar runs for roughly a couple of days on a full charge — plenty for a real search, but not something to leave running.

Should I wait for the next generation before buying? Waiting on an unannounced product is usually a bad trade, and Fi’s upgrade policy for active subscribers softens the risk of buying now. The more useful question is which platform you want to be on. If you’re weighing Fi against the other major cellular tracker, our Fi collar vs Tractive head-to-head compares them on subscription structure and coverage rather than generation.

How does the Series 3+ compare to the rest of the market? It’s a premium-tier collar built around durability and escape response, with health monitoring bolted on. Trackers built primarily for health data or for cats take different shapes entirely — see our 2026 GPS dog tracker roundup for where Fi sits against everything else, and our full Fi Series 3 review for the long-form take on the platform.

Conclusion

The Series 3 to Series 3+ jump is a mid-cycle refresh that punches above its name. Same collar, same bands, same size range, same waterproofing — but with trained AI reading your dog’s behavior, GPS that locks on roughly twice as fast, and a couple of quality-of-life additions in the app.

If you’re buying today, the Series 3+ is the answer, because it’s the only one Fi still sells new and there’s no Series 4 waiting in the wings. If you already own a Series 3, the honest advice is to keep it, keep your subscription active, and let the upgrade eligibility work in your favor when the next generation finally lands.