Single-Lever Valve Does Not Shut Off

I have a single-lever valve for my washing machine. It is manufactured by Symmons (W400 or W500 - http://www.symmons.com/products/res_product_detail.cfm?id=401&category=9&function=24). It is only 2 years old and it was installed by a professional plumber. However, the valve does not shut off when I move the lever to the off position. Is there something a DIYer can do, or should I call the experts?

Thanks in advance!

Its a relativly easy fix... !st get the exact same valve. turn off the water to the house and check that it is off. Remove the 2 stainless steel screws holding the old valve on the elbows. Put the new seals and valve on tighten the screws and turn on the water again. Install a set of stainless steel no-burst hoses while your at it.

$38 and about 10 minutes later (most of the time spent turning off the main water supply) works like a charm! Thanks for the tip!