The Karcher HD3000 is considered by Karcher to be a "Professional" user model, it is not a bad unit for the price, we sell it on-line at:
Karcher HD3000DH it is a nice little cold water washer powered by a 9HP Honda GX series commercial engine driving the Karcher pump, rated 3.2 GPM @ 3000 PSI.
The injector is rebuildable on most Karcher models and I assume the same for the HD3000 models, as it is an integral part of the pump.
If you e-mail me the complete model # and serial # I can get you a rebuild kit for the injector.
It sounds like it is probably under warranty though, you can probably get it repaired for free by an Authorized Karcher Warranty Repair Center.
Karcher Warranty Program
Best Wishes!
If you were in my neck of the woods , I would tell you bring it and we will fix it for you.
Check out the Karcher link above, I am sure they will help you out!
I have a customer with a cold HP Karcher that just keeps running for years, I prefer slower RPM belt driven pumps myself.