What I want it for is not as a substitute for professional maintainance but so that I can be sure the MIL isn't indicating something other than the existing minor problem. It's a known problem amongst Previas of that age and dealers are 'fixing' it by replacing all 4 sensors and the cat as well as reprogramming the ECU! There's a really long thread about it on the Toyota Owner's Club forum.
Warrenty's expired of course
