p0401. Fault code. Have had the valve cleaned but fault came back need advice?

Can I check this myself? As I live close to only one garage and they are charging me and not fixing the problem. Carbon build up in the ports? Is this on the valve? I've ordered a second hand valve.

Thanks for reply I've replaced the valve with a second hand part. It seems withe the garage did not clean the valve or I have serious engine problems. As I don't think it would take one week for so much carbon to build up the the entrance hole to be almost blocked.