In pressure gauge if the throttle valve is not working or not showing exact reading how to rectify?

First of all as per standards double valve isloation should be available in steam duty or any other high pressure medium even on feed water. 

First of all isolate the PG and then flush the line. The PG should read zero if there is zero error then calibrate the PG. then once again deisolate the PG it will read accurately.

FIRST ISOLATE THE GAUGE AND FLUSH IT. IN CASE OF ANY READING ERROR, DO ZERO ADJUSTMENT AND CALIBRATE THE GUAGE.

You would need to use "Zero adjustment screw" to make it zero and clean the manifold, calibrate again.

Calibration or adjustment is enough to fix the deviation of readings  

 isolate pressure gauge and then flush it.if still not showing correct valve dismantle and re

 calibrate.

isolate that pressure gauge and then flush it upto zero pressure value.

If the throttle valve is not working and not showing exact readings connect the pressure valves to Bourdan Guage and pump for two air pressures and the throttle valve get resetted and start your calibration.

connect gauge with standard pressure and check out its milliAmps. and calibrate it by adjusting screw.

First of all isolate the PG and then flush the line

The pressure gauge should read zero

if there is zero error, then, calibrate the pressure gauge