2000 Rav 4, engine light

I have a 2000 RAV4 and though I am happy with it's performance the check engine light keeps coming on. I bought the car used in the summer of 2004, in September of that year the light came on for the first time. I took it to Toyota (Atrell) and they charged me over $500 to replace a valve. In November 2005 the light came on again, then again in January 2006. At that time I took it back to Toyota (service) and had the Knock sensor replaced for over $700, two nights ago the light came on again. It's only been 5 months since I took it in and had it fixed. When I called the service department at Toyota they told me that their diagnostic system could only give a range to look in and not a specific part to fix. So my question is if they don't know exactly what they need to fix to turn the light out why did they replace those two very expensive parts in the first place and how often is the light going to come on??