Johnson Controls A 4000 121 Oil Indicator Els PW 17895 Optimized Motor Torque: Maintains the Optimized Motor Torque: Maintains the necessary phase shift to keep HVAC compressors running at peak efficiency Internal check valves offer low pressure drop while maintaining 40 micron filtration efficiency The Johnson Controls A 4000 121 is a critical pneumatic oil indicator replacement cartridge designed for high performance compressed air treatment in pneumatic control systems High oil readings typically indicate that the coalescing or activated charcoal filters in the A 4000 series station are saturated and require immediate replacement to restore air quality