（1） Pneumatic control valve does not operate
The fault symptoms and causes are as follows:
1. No signal, no gas source. ① The gas source is not turned on, ② due to the water content in the gas source freezing in winter, the air duct is blocked or the filter and pressure reducing valve are blocked and malfunctioning, ③ compressor failure; ④ The gas source main pipe is leaking.
2. There is a gas source but no signal. ① Regulator malfunction, ② signal tube leakage; ③ locator bellows leakage; ④ damaged regulating mesh plate.
3. The locator has no air source. ① Filter blockage; ② Pressure reducing valve malfunction (③ pipeline leakage or blockage).
4. The locator has a gas source but no output. The throttle hole of the locator is blocked.
5. There is a signal but no action. ① Valve core detachment, ② valve core jamming with society or valve seat; ③ The valve stem is bent or broken; ④ Freezing of valve seat and valve core or burnt debris: ⑤ Rust death of actuator spring due to long-term disuse.
（2） The action of the pneumatic control valve is unstable
The fault symptoms and causes are as follows:
1. Unstable gas source pressure. ① The compressor capacity is too small; ② Pressure reducing valve malfunction.
2. The signal pressure is unstable. ① The time constant (T=RC) of the control system is inappropriate: ② The regulator output is unstable.
3. The air source pressure is stable, and the signal pressure is also stable, but the action of the pneumatic regulating valve is still unstable. ① The ball valve of the amplifier in the locator is contaminated with dirt
When the wear is not tight and the gas consumption increases significantly, output oscillation may occur: ② The nozzle baffle of the amplifier in the locator is not parallel, and the baffle cannot cover the nozzle; ③ There is air leakage in the output pipe and line; ④ The rigidity of the actuator is too small; ⑤ There is high frictional resistance during the movement of the valve stem, and there is obstruction at the contact part.
（3） Pneumatic control valve vibration
The fault symptoms and causes are as follows:
1. The pneumatic control valve vibrates at any opening. ① Unstable support; ② There is a vibration source nearby; ③ The valve core and liner are severely worn.
2. The pneumatic control valve vibrates when approaching the fully closed position. ① The pneumatic control valve is selected too large and is often used at a small opening: ② The flow direction of the medium in the single seat valve is opposite to the closing direction.
（4） The action of the pneumatic control valve is sluggish
The phenomenon and reasons for sluggishness are as follows:
1. The valve stem is only sluggish when moving in one direction. ① Membrane damage and leakage in pneumatic membrane actuator; ② Leakage of O * type seal in the actuator.
2. The valve stem exhibits sluggishness during reciprocating motion. ① There is adhesive blockage inside the valve body: ② The polytetrafluoroethylene material has deteriorated and hardened, or the asbestos material lubricating oil has dried; ③ If the packing is too tight, the frictional resistance increases: ④ Due to the non straight valve stem, the frictional resistance is high; ⑤ Pneumatic control valves without locators can also cause sluggish operation.