I'm getting a P1647 code, but no noticeable change in fuel economy and no check engine light. There's plenty of threads about this code and the O2 sensor but I can't find any that don't also mention the CEL being lit. Mine isn't - could it be anything else than a busted O2 sensor?

 

So long as your MIL works then you're not seeing a current code and can ignore it.

Your tool SHOULD make this clear!!

 

Hmm. If I clear the code though and re-scan, it goes away. When I go for a drive, it comes back.

 

First, stop clearing it. It's not helping you.

Then do some warm ups, probably different days.

Chances are you'll get the code & MIL in which case you need to FIX the car.