Something after the compressor is blocking airflow -- a frozen dryer, closed valve, or severe filter blockage. Pressure has nowhere to go.

What you'll see

Safety valve pops, but the issue is downstream of the compressor. On machines with integrated dryers (like Atlas Copco GA FF series), the pressure sensor may be located AFTER the dryer. If the dryer freezes shut, the sensor reads low pressure while sump pressure skyrockets because the controller keeps loading, thinking pressure is low.
Before you assume this is the problem

If the safety valve pops with the outlet fully open and good airflow to the system, the restriction is inside the compressor (separator) not downstream.

See all causes of safety valve blows / too high pressure →

How to diagnose

  1. Check the dryer temperature

    On machines with integrated refrigerant dryers, check the dryer evaporator temperature. If it's below 0 degrees C, the evaporator may have iced up, completely blocking airflow. This is a known failure mode on GA FF models.
    Result: Dryer temp below 0 degrees C = frozen evaporator.

  2. Bypass the dryer

    If the system has a dryer bypass valve, open it. If pressure normalizes and the safety valve stops popping, the dryer is the restriction.
    Result: Normal operation on bypass = dryer is blocked.

  3. Check for closed valves

    Verify that the outlet valve and any downstream isolation valves are fully open. A partially closed valve creates a restriction that raises sump pressure.
    Result: Partially closed valve found = open it.

How to fix it

  1. Fix the dryer

    If the evaporator iced up, the hot gas bypass regulator has likely failed. This valve prevents the evaporator from getting too cold. Replace or repair it. Let the dryer thaw before restarting.

  2. Replace severely clogged downstream filters

    Inline filters downstream of the compressor can clog to the point of creating significant backpressure. Replace filter elements and check that housings are not damaged.

Common mistakes

On machines where the pressure sensor is after the dryer, the controller has no idea that sump pressure is climbing dangerously. It keeps loading because it reads low pressure at the sensor. The only protection is the mechanical safety valve. This is a design limitation worth being aware of.

Parts & tools

Dryer hot gas bypass regulator. Replacement inline filter elements. Temperature gauge for checking dryer.

Review safety precautions before starting →

This issue can also cause

  • Pressure Drop Significant pressure loss between the compressor discharge and point of use, causing tools to underperform des...