After using the P4 screen to set up temp control, and calibrate my pH probe, I need to recalibrate the pH probe due to faulty high reading. Now the screen on the P4 will not respond and I am unable to use the down arrow. With repetitive touching, I can get the X and check to work, and occasionally the left arrow but those get me no where. I did unplug the P4 and plugged it back in to reset it but no change.