Master DADoESTX has written up an excellent explanation of what the diverter valve does in these washers, the problems it can cause, and how to test it:

The wash period (Heavy, Normal, Delicate, & Woolens cycles ... not Perm Press) has two different wash processes. The first part is the EcoActive wash. The tub fills with just enough water to saturate the clothes and a couple/few gallons more, then the pump turns on and concentrated detergent solution sprays over the clothes for about 5 mins while the basket rotates at 25 RPM. Recirculation is by way of a diverter valve that shifts the pump's flow from going out the drain hose to going through the recirculation hose and back into the tub. If the valve is not functioning, shifting into recirculation mode, then the water will drain when the pump turns on, and the machine will keep filling in an effort to maintain the required water level. It should trigger a fault code on this situation. Note that there may be a little more filling occur after the pump recirculation begins to top-off the water level for what's "in the air" for recirculation.

After the EcoActive wash function is finished, the tub fills rest of the way for an agitated wash, during which the pump does not run.

Perm Press does not run the EcoActive wash, goes directly to the deep-fill agitated wash .... so the times when your machine is working OK may be when Perm Press is being used.

You can test the diverter valve via Diagnostic Mode.

- With Power off, press and hold Wash Temp Down, then press Power at the same time. Should get two beeps and the panel activates into Diagnostic Mode.

- Press Delicate to activate the diverter valve. It's a wax motor that takes 1 to 2 mins to heat and shift into recirculate mode. When deactivated it takes 2 to 3 mins to cool and shift back to drain mode. There's no visual indication on the panel when it shifts, have to turn turn it either on or off and wait the requisite time.

- While waiting for the diverter to shift, fill the tub with enough water to see it start coming up the agitator base. Wash Temp Down runs cold water or Wash Temp Up runs hot water, your choice.

- Assuming a couple mins have passed and the diverter has had time to shift, now press Regular to turn the pump on. Water should shower out of the recirculation port at right-rear top of the top, and NONE from the drain hose.

- Press Delicate and Regular to turn the diverter and pump off. Wait a few mins for the diverter to cool and shift as explained above. Press Regular to turn the pump on again, and water should flow out the drain hose and NONE from the recirculation port.

If either recirculation or drain does not work properly (but the pump does run), then the diverter valve is either bad or clogged with debris, blocked from shifting modes.

This procedure for operating the diverter valve via the Delicate cycle button in Diagnostics is not outlined in the service material that I've seen for GWL11, GWL15, etc., but it does/should work. It is stated in the GWL10 service supplement. (The diverter on IWL12/15/16 can be operated via the Home button [the LCD panel will say Diverter Off or Heating or On or Cooling], and the pump via the Fabric Care button.)

Source: Fisher Paykel GWL11 - Intermittent Issues with Cycle