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Shower Faucet: Low Flow-M-Core 4 Series Valve

The most common reason for low flow in a shower the cartridge. See the chart below for troubleshooting help.

What type of Shower System do you have?

Standard Shower or Tub/Shower


Vertical Spa using a Transfer Valve


Mixing Valve: The 4 series is a unique shower system in that it has two handles that are stacked in front of each.

1 Flow Handle: This handle turns the water on and off and controls the volume of water.

2Temperature Handle: Operating this handle allows you to set your desired water temperature.


Transfer Valve: The M-Core series transfer valve numbers are U231 or U361.  

3-Transfer Valve Handle: Rotating this handle activates the individual showering device. Example: Showerhead

4-Shower Device: Example: Showerhead, Handheld Shower

Which part of my system is causing the problem?

If you have a Vertical Spa (Multiple Device System) and one of the showering devices is not operating a full flow of water. Confirm all of the other devices are working properly. If they are, see Vertical Spa: Independent Transfer Valves, for additional information, this indicates an issue with the Transfer Valve.

For all other situations and symptoms continue below:

Symptom Solution  
Low Flow: Temperature Specific

A reduction in flow when operating one specific temperature. (Ex: Only when using hot water)

Replace the Thermostatic valve cartridge, see article: 1214 M-Core Cartridge.

Low Flow: Hot and Cold

A overall reduction in flow from a showering device.



1. Check for blockage in the shower device. The low flow symptom can occur if debris has come through the line and became trapped causing a reduction of flow.

Remove the device and turn the water flow on.

Is the flow of water normal with the shower device removed?

  • Yes, rinse any debris from the shower device and re-test the flow. If the symptom persists, replace the shower device.
  • No
    • Single Showering Device replace the Thermostatic valve cartridge, see article: 1214 M-Core Cartridge.
    • Multiple Devices replace the Diverter valve cartridge 
No Flow

There is no hot or cold water from the shower system. 

  1. If the shower was recently installed or serviced, confirm the check stops are fully open and both of the water supplies to the valve are on.  
  2. Replace the Thermostatic valve cartridge, see article: 1214 M-Core Cartridge.
All Hot or All Cold Water




The M-Core 4 series utilizes a Thermostatic cartridge. If the system is only providing extreme hot or extreme cold (no mixed water-warm), there are two common causes.

New Installation:

1. Cartridge is backwards (hot and cold are on wrong sides). Try flipping cartridge 180*

2. Replace the mixing valve cartridge, see article: 1214 M-Core Cartridge.

Existing Installation:

Replace the mixing valve cartridge, see article: 1214 M-Core Cartridge.

For additional information on M-Core Shower products, visit Two Handle Shower: M-Core 4 Series at

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