Hy frnds, i am trying to study electronics trouble shooting, i have gas analyzer wich shows measurements in local display and at remote display through 4 to 20mA signal, problem is that it's zero is totally out, it shows 4% at nil gas detection, but analog signal is ok as 4mA at nill gas detection, i tried my best to set zero by adjusting both zero and span potentios but it stuck at 4%, what might be the problem

 

Hi Saadi,
Do you have a circuit diagram of the unit that you could post.?
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