Im getting a flow error at 800 flow, dropping it down to 400 the problem goes away, what could the problem be?

What is a torn balance chamber membrane? Chuck
A small tear in the membrane may not be noticed at lower flows but at higher flows can cause flow errors. Check you balance chamber volume both fresh and spent and go from there.

Good luck

I should have been more specific Im getting a low flow error

Did you check your pump pressures? Dearation, Flow?? Inlet pressure? A torn membrane can cause this as well. Check your hydroblock orific, make sure it isn't plugged. What kind of machine are we talking about anyway?

2008k fresenius machine,I checked the pressures but I will check the hydroblock and membranes , Thanks

Is this a constant flow error or intermittent? You can also do a valve leak test. Check your valves on the flow screen and make sure they are all operating. May be a bad pump head? It may work fine at lower flows but can't quite handle higher flows. It could be a lot of things. Best thing to do is follow the troubleshooting guide for flow errors.

May be CFS or worn brushes to your pumps. If your machines have over 8000 hours, it might be time to clear the dust and replace the brushes.