I know there's a few threads around relating tangentially to this, but hopefully someone can answer my question directly:

I purchased a 2007 G35x almost two years ago now with 93k on it. I'm now at 107k. I cleaned my throttle body last summer (it was in pretty bad shape) and cleaned the MAF sensor while I was at it (RE: https://g35driver.com/forums/v36-diy...le-bodies.html).

Ever since, though, I've been getting horrible gas mileage, lately being around 18-19mpg with 100% highway (driving to/from home for the holidays, 600 miles total, granted I was driving 78mph on cruise control) and 14-15mpg 50/50 highway. It's cold up here, but I even so. Another point is that when I turn the car on (even on 90F summer days) it idles around 2k rpm before slowly, in 100 rpm jumps, settling down to around 1100 rpm. After a few minutes of driving it'll finally settle out at 800 rpm or so.

I have gotten random CEL (O2 sensor) calls over the last year but they go away on their own. Would a bad/dirty O2 sensor be causing such bad mileage? I'm assuming it's worth cleaning them before replacing in either case and I couldn't find a DIY.

 

It could very well be your sensors. Is it always the same one, or do the codes come from multiple sensors? Have you done thorough research on the codes you did get? IDK if you'll have much success with cleaning the O2 sensors though. Browsing the interwbz a bit, there seems to be conflicting opinions on the overall efficacy of cleaning them. Some report good results, while others claim that even when the best practices for success are employed, the likelyhood of potentially damaging them remains high. I guess it couldn't hurt to try, but remember, it might just be tme to pony up for replacement. With careful shopping, they're not terrribly expensive. Before I did any of that though, I think I would try cleaning the MAFs again too, just in case something went wrong there. Are your air filters in good shape too?