colorado-yota

My V-8 Four Runner has 150,000 miles on it. My gas mileage has dropped about 4 mpg in city and on highway. The engine is otherwise performing great and has plenty of power.

Wondering if the O2 sensors might be getting tired. Can I rely on the check engine light to let me know? Or should I test the sensors? Or should I just go ahead and replace them?

I have kept it up on the routine tune-up items, but there could always be a cracked vacuum hose somewhere that I missed.

Anyway, any thoughts?

j2the-e

You don't have o2 sensors. Your truck has a/f ratio sensors. They can absolutely be misbehaving and not setting a check engine light. I would get a good scanner that reads live data and look at the long and short fuel trim values. Add the trim #s together(bank 1 separate from bank 2) and if they are positive 15-25% it is dumping extra fuel but not enough to set a check engine light. Is it enough to cause a 4 mpg decrease while not setting a code? Very Doubtful.
Also, it won't hurt to check your tire pressures, clean the MAF sensor, clean the throttle body and make sure the maintenance is up to date. A broken vacuum hose would most likely set a lean code. A vacuum leak will also cause positive fuel trim #s.

2 other questions: 2WD, AWD or 4wd? What is your prior and current mileage?

Red3.slow

All of what is mentioned above is good stuff. I replaced mine recently and did notice a slight improvement with economy.