[george renwick]

I use two vessel system and the first one is only reading 4  but the second in l one keeps coming out at 1 or 2  .Is there anything inside them that can go wrong and needs to be replaced 

[Shrek]

Could be your tds meter? Is it a cheap one off eBay?

[JandS]

Not necessarily off e bay I've had one from a reputable supplier that reads wrong........

[JandS]

Altogether I have 3...all the same make and not one of them reads correctly with the calibration fluid....probably the fluid although that was from supposed reputable supplier.

[Slacky]

The riser tube can either be loose or leak, maybe from a crack in it. This can mean water bypassing the resin leading to contaminated water.

[dazmond]

I use two vessel system and the first one is only reading 4  but the second in l one keeps coming out at 1 or 2  .Is there anything inside them that can go wrong and needs to be replaced 

Its obvious.......your second vessels resin is spent....