How do I narrow down reason sequoia won't start?

Hello, P0420 and P0430 themselves do not indicate a failed component that is preventing the vehicle from starting. But it sounds like evidence of excessive fuel being dumped into the engine historically, causing an issue with the catalytic converters - as extra fuel can make them so hot they become damaged and in turn prevent them from being functional efficiently. This will turn the light on. As far as a cranking with no start, the engine could be getting excessive fuel or not enough fuel; could be getting no spark, could have an air problem with not enough or too much air hitting the intake manifold. Too many "could's" to help - need hands on diagnostic testing. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops