The supply flow is great. The flow is low even with the spray head off. The hose coupler is OK according to Grohe and the flow is low with the coupler off. Therefore, the problem should be in the cartridge area. Any experience repairing these faucets?

Hello Sommerfield,

See the image below is this your faucet, if it is you need to remove the handle the cap and you will get in to the cartridge, if it is not your faucet please post the model number, Thanks.

John

By saying "hose coupler" you mean the back flow preventer ?