You can a) adjust to ATM by pushing down the ADJ button for 5 seconds (make sure to have the gauge on and at ATM) and b) take the gauge apart and measure the pin resistances. If the resistance is too high, you will need to replace the sensor tube. Instructions on how to check the pin resistances can be found in the manual.
Category: Instrumentation Gauging
Sub-Category: Wide-Range Combination Gauges