zxredline

In Summary: 99 RX300, 160k+ miles

~18mpg down to ~14mpg, mixed city/hwy
All Tires inflated to 36psi
No Codes

RX runs well otherwise, no misfires.

- Fixed exhaust leak by cat (new)
- Cleaned air sensor
- Air filter looks new
- Knock sensors replaced

It appears that all highway miles is great (~20mpg) but mix a bit of city driving, mileage just plummets down to lower teens. I drive very conservatively. Don't think this is a tuneup issue, since the mpg dropped quite suddenly from a very consistent ~18mpg to ~14mpg.

The only thing I can think of are O2 sensors or vacuum/exhaust leaks? Can O2 sensors fail without throwing a code? If so, how many are there to change and is it possible to pinpoint which one is failing, rather than changing them all out?

Thanks for any info in advance.

hypervish

Welcome to Club Lexus!

Have you cleaned the MAF?

Winter blend fuel can have a huge impact on fuel economy, I generally see a 3 mpg loss during winter.

mckellyb

I've had O2 sensors get flaky without throwing a code, though not on my wife's RX.

160K miles...it's overdue if they're original.

Nomjisly

I have a similar problem. I have read the replies and will check out those areas mentioned. Hope it works!