I have twin 2001 Yamaha 200 HP HPDI with 525+ hours each. The starboard motor will not go over 5000 RPM. The local Yamaha techs say O2 sensor is dirty and asks "Have you been using ring free?". What's Ring Free? And how come nobody told me to use it? I've been reading this (excellent) forum, and I surmize I must shock with ring free, and use ring free and replace the O2 sensor. Of course I will replace the O2 sensor in each motor (wonder how much they cost?). Is this the correct plan of action?