I have an intellichlor IC20 and had the same issue for a very long time. I acid washed all the time and eventually had to have the unit replaced (under warranty) 2 times. Pentair didn't even know what was wrong, they just kept replacing it and sure enough after a few months the same error would come back. Anyway, I finally figured out that the issue was high phosphates. If you already haven't done this take a water sample into your local pool supply store and have it tested. Make sure that you ask to test for phosphates because they probably won't unless you ask. If there are any phosphates present you need to use a product like PhosFree to remove them. I had a phosphate level of 2000 ppm (normal range is 0-200) so they recommended the commercial strength PhosFree which was $60. I used half the bottle (make sure to shake it well before pouring in) and then waited 3-4 days before taking another water sample back in to be re-tested. This time it was at 1000 ppm but the important thing is that the check flow/pcb error was gone and the salt level reading seemed to be within a close range of the actual salt level in the pool. I used the rest of the bottle to remove the last 1000 ppm and re-tested after another 3-4 days. Phosphates were under 100 ppm and the salt reading on the easy touch was spot on with the salt test strips that I use.
I found out that jacks magic (purple stuff) was the main culprit. It has phosphates in it and apparently phosphates coat the sensor and can't be removed with acid washing. I now use a product called Metal Free by the same makers of PhosFree since it has no phosphates in it but I think jacks magic also makes a phosphate free product as well.
Recently though I started noticing that the salt level on the easy touch started to drop off little by little until it got down to 2500 while the actual salt level was at 3400. I went back in to the pool store to test for phosphates and they were only at 175. The guy told me that anything under 200 is fine and not to worry so I called pentair and they told me that the unit is very sensitive to phosphates and that you basically need to remove all phosphates in order for the unit to work. I wish they had told me this before I had the unit replaced twice. Even a small amount of phosphates will coat the sensor and over time it will build up. The only way to fix it is to remove ALL phosphates and keep them out. They recommended a maintenance of regular PhosFree to make sure that there isn't a build up of phosphates (apparently tap water even has around 100 ppm). Basically get your water tested and if there is any phosphates present, remove them and keep them out. Don't mean to ramble on but this drove me crazy for the better part of a year so hopefully this will help you out so you don't go through the same thing.
Good Luck,
Steve