Every holding tank and septic system is a little different, but the general advice here works for most. A holding tank sender is a simple device with nothing to wear out, and with a bit of routine care it will keep working reliably for many years. When one does misbehave, the cause is almost always the same - and usually easy to put right.

01 / CAUSESWhy a holding tank sender stops reading

Problems nearly always come from one of two things. The first is a sender that's too long, so its lower end sits in the bottom couple of inches of the tank - exactly where the heaviest solids collect. The fix is to fit a shorter sender and leave at least 2 inches (50 mm) of clearance at the bottom, keeping the sender clear of that layer.

The second is a tank that's been emptied and left standing without the sender being rinsed. Paper is often the culprit: as it dries it sticks to the float and stops it moving freely, so the reading no longer changes. The simple solution is to rinse the sender before it's left to dry out - which brings us to routine care.

02 / CAREA yearly rinse keeps it sweet

The single best thing you can do is rinse the sender with a hosepipe whenever you empty the tank, and before laying the boat up for winter or any long spell unused. As good practice, give it a rinse at least once a year, and always before an extended period standing empty - that's when dried-on paper and solids do their damage.

To make access easy, an FLB inspection hatch - or the FLB-1 inspection hatch with sender fitting - lets you get to the tank and sender for cleaning without cutting more than one hole.

03 / REVIVEBringing a stuck sender back to life

If a holding tank sender has stopped working, don't write it off straight away. The first thing to try is to remove it from the tank and stand it in a bucket of hot soapy water for a few hours, then rinse it off with fresh water and see if it works again. More often than not, softening and washing away the dried-on residue is all it takes to free the float and get an accurate reading back.

04 / THE CAGERemoving and testing without it

The protective cage on the S3H sender is held on by a single spring clip at the bottom. Remove the clip and the cage simply slides off the stem, so you can rinse or inspect the sender easily.

If you're having trouble with the float sticking on your particular septic system, it's worth testing with the cage removed completely. The sender works correctly with or without the cage, and many OEM boatbuilders prefer to run their holding tanks without it. With proper maintenance, either way, the sender will go on working for many years.

05 / SHOPSenders and access

06 / FAQCommon questions

Still stuck? See our upgrade guide, or get in touch - tell us what you've got and the internal depth of your tank, and we'll help you get it reading right again.