Trouble with my Fluval FX4

I bought an 8 month-old Fluval FX4 filter bundled with an aquarium second-hand on FB Marketplace. After setting up the tank, getting the filter going wasn't much of an issue; It did take a long time (a couple of hours) to transition from its priming cycle (2min off, 1min on), but once it had settled in it ran without any problems. ... at least that was until a few days later when I did the first water-change.

After the first water-change, the FX4 would not leave "priming mode". For days it cycled; 2min off, 1min on. It was enough to keep the tank going (it's only 200L), but clearly needed seeing to. At first I thought maybe the hoses were fouled; So I cleaned them. Then I thought maybe the filter media arrangement was restricting flow too much or something; Removed some of the bio-media. Still not working, so I removed even more filter media; still the same. I removed and cleaned around the impeller (again). A little sand was present. I found that the ceramic bio-balls were hiding sand, so I removed all of them, thoroughly re-washed all the media, and tried again. Still no joy. This went on for weeks.

I replaced the impeller assembly. Cleaned and greased all of the O-rings. Shortened the inlet and outlet hoses. Raised the filter so that it was closer to the tank water-level. Double and triple-checked for any impediments to flow. Likewise for air-leaks. Flow was fine during the 1min that the pump was on (out of every 3min). It just wouldn't get out of "priming mode".

I asked around online. Chatted to the local supplier. Considered a warranty claim (not really an option since it seems that the sand had initially partly fouled the impeller). But despite everything being apparently A1, the pump was still off for 2 out of every 3mins. I considered chucking in the towel and accepting that the off/on cycle could be a good thing (cycling flows can be cool). But it still bugged me. I wanted the best start for my new tank, and this was far from ok.

In the end I gave up and spent AU$250 on a replacement pump. Add to that the price of the new impeller assembly, and you could buy a very good filter for a 200L tank new. But now it's fixed, and I have a fantastic option for a low maintenance, and extremely effective filter. If I had more spare time, I might have experimented more with trying to reset the pump. My theory is that, with the sand fouling the impeller, the "sensed motor parameters" for driving the pump were shifted out of the viable range by some kind of "smart" self calibration. If I could have reset the programming, I think it would work again. I hate stuff that tries to be smart when simply off and on are really all that's needed; more failure points for small gains.