What a bummer, I'm so sorry!

Call or email Grohe and complain. They have a lifetime warrant on their faucets as long as you are in the home and have the receipt. I've had Grohe faucets for years in my baths and never a problem, but I've heard they are very responsive on these issues. At the very least they should reimburse you for the parts you had to replace.
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Grohe Warranty

Limited Lifetime Warranty (U.S.A. Only)

Residential Products: Grohe provides the following warranties on its products to the original purchaser, installed in a residential application. This warranty is effective for all faucets sold after January 1, 1997.

Mechanical Warranty: A Limited Lifetime Warranty is provided on all mechanical parts to be free from manufacturing defects in materials and workmanship under normal use for as long as the original purchaser owns their home.

Finish Warranty

General Finish Warranty: A 5-Year Warranty is provided on all GROHE product finishes (except the InfinityFinishTM) to the original purchaser against manufacturing defects in materials and workmanship.
InfinityFinishTM: A Lifetime Warranty is provided on all GROHE products with the InfinityFinish to the original purchaser against manufacturing defects in materials and workmanship.
At its option, GROHE will repair or replace any part or finish that proves to be defective in material and/or workmanship under normal installation, use and service.

To obtain warranty repair or replacement the faulty part or product is to be sent along with proof-of-purchase (original sales receipt) from the original consumer purchaser to:

GROHE America, Inc.
241 Covington Drive
Bloomingdale, IL 60108
Telephone inquiries can be directed to: (630) 582-7711

This warranty is extended to the original consumer owner, but does not cover installation or any other labor charges and shall not apply to any products or parts of products which have been:

a) Repaired by unqualified persons or not using original GROHE spare parts or altered in any manner unless previously authorized in writing by GROHE; or

b) Damaged as a result of misuse, negligence, accident or faulty installation or maintenance; or

c) Installed or used in a manner contrary to GROHE's printed instructions; or

d) Damaged by the effects of dirt, limescale or aggressive cleaners.

This is the exclusive warranty by GROHE, which does not make any other warranty of any kind, including the implied warranty of merchantability.

This warranty excludes all industrial, commercial, and business usage whose purchasers are extended a one (1) year limited warranty from the date of purchase, with all other terms of this warranty. (See following for commercial products.)

Commercial Products: Grohe warrants its products installed in a commercial application to be free from manufacturing defects for a period of one (1) year from the date of acquisition by original owner. Replaced products or parts are warranted only for the period remaining under the initial warranty.

THIS WARRANTY IS LIMITED SOLELY TO THE ABOVE AND THIS WARRANTY AND ANY WARRANTIES IMPLIED BY STATE LAW WILL APPLY ONLY FOR THE APPLICABLE WARRANTY PERIOD SPECIFIED HEREIN. ALSO, GROHE WILL NOT BE LIABLE FOR ANY LOSS, DAMAGE, EXPENSE OR INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, WHETHER BASED ON WARRANTY, CONTRACT OR NEGLIGENCE AND ARISING IN CONNECTION WITH THE SALE, USE OR REPAIR OF THE PRODUCT. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF CONSEQUENTIAL DAMAGE OR LIMITATIONS ON HOW LONG AN IMPLIED WARRANTY LASTS, SO THE ABOVE LIMITATIONS OR EXCLUSIONS MAY NOT APPLY. THIS WARRANTY GIVES YOU SPECIFIC LEGAL RIGHTS, AND YOU MAY ALSO HAVE OTHER RIGHTS WHICH VARY FROM STATE TO STATE. UNLESS OTHERWISE CONTRARY TO STATE LAW GOVERNING THE PURCHASE, GROHE'S LIABILITY WILL NOT EXCEED THE CONTRACT PRICE FOR THE PRODUCT CLAIMED TO BE DEFECTIVE OR UNSUITABLE.

Thanks Sharonite, we are in Canada and did take it back to where we bought it-they will reimburse us the money when we bring back the faulty cartridge. They did give us the phone # for the Grohe representative in our area who basically just laughed when we called him and didn't seem to care-just said to deal with where we bought it. :(

It's installed now with the new cartridge so hopefully it won't leak any more-just was disappointing when you pay so much for the product to leak after only a couple of months.

Grohe claims they pride themselves on making their own cartridges.
Most faucet manufacturers buy cartridges from places like Sedal and Hydroplast.