Unable to empty waste tank

I have been told that the various gauges are a guide and not 100% accurate but my waste tank shows 25% full all the time.  New waste water does empty and I had assumed that it was because I was not on the level at home but I have just been away for a week perfectly level but it still constantly showed 25% full.  I was even guilty of leaving my drain tap open on the journey home but no joy !

Hi Bill, I have had this problem myself both with my old Gatcombe and my Kingham now. What happens to mine is the sender from the tank needs resetting to empty. 
I'm not sure how you can do this on your Symbol but it is in my manual so maybe you can have a read before bed tonight! 
Happy camping:boat:

If you open the drain valve while reasonably level, does anything come out?
If it does does it eventually stop
If either nothing comes out or does so and stops, then the tank is empty.
Virtually all tank level gauges except visual ones are works of fiction IMHO.
Therefore for a simpler life, drain the tank sensibly and ignore the gauge!!!!

My 2017 Symbol was doing similarly (at the end of a VERY long trip to Europe)  but a thorough clean with Thetford tank cleaner (and driving it around for a few days with a good dose of said cleaner) seemed to sort it. Fairly sure that it will happen again but sorted for now.

I've had the same problem but found cleaning & de-scaling the tank generally cured it.  I'm told the external sensor connections can also cause it if they get wet from road spray.  I've caked mine with silicon sealant.

IanH wrote:If you open the drain valve while reasonably level, does anything come out?
If it does does it eventually stop
If either nothing comes out or does so and stops, then the tank is empty.
Virtually all tank level gauges except visual ones are works of fiction IMHO.
Therefore for a simpler life, drain the tank sensibly and ignore the gauge!!!!

Apparently the tank works as is should and fills up and can be 'emptied' by opening the tap but the gauge never goes back to zero but stays at 25% as if there is waste water left in there.  Bill.

I’m with Ian on this, get a feel for your typical use (both waste and fresh) and ignore the gauges. After the novelty of having gauges for the first time ever in years of camping wore off, I haven’t bothered with them.
I realise many find them useful but from all the similar threads on here they are nothing but trouble!

Bill, ignore it. Tank gauges are notoriously inaccurate and I think Ian's got it right.

IanH wrote:
Virtually all tank level gauges except visual ones are works of fiction IMHO.
Therefore for a simpler life, drain the tank sensibly and ignore the gauge!!!!

 Hear Hear!

Cymro

Ours keeps staying at 25%. When we got home after our latest tour I left the tap open and it is now reading properly at 0%.