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Bad O2 sensor w/o check engine light?

Is it possible to have a bad oxygen sensor without the check engine light coming on? My mileage as dropped by a LOT over the last couple of months. I’ll be changing the spark plugs and PCV valve this weekend, and if that doesn’t help, borrow the vac test kit from the parts store. I think that would leave the O2 sensors as the only other culprit except for the lack of a check engine light.

I have been wondering about the same thing myself, as exhaust smells a bit of unburned fuel at start on cold days. I was going to see how the next emissions test worked out.

Long before I worried about an O2 sensor I’d make sure of other basic things. Here’s a basic list:

  • tire pressure and tire condition
  • thermostat and coolant temp sensor
  • alignment
  • check of brakes for any dragging
  • air filter & fuel filter

In general (don’t know about your Dodge in particular), there are plenty of other things that it could be. Fuel pressure, MAF, MAP, coil(s), fuel injectors, transmission issues, and on and on. Any marginal condition or sensor might not trip a check engine light.

In response to that all my basics are up to date.

My mileage is down by about 50-60 miles per TANK! about 3 MPG per gallon. That is way more than can be accounted by something simple as tire pressure, alignment, coolant, etc (and they are checked, BTW). And this low mileage has happened in the last couple of months, not a long slow degradition, which is why I suspect the things I mentioned in my OP.

I’ll have to disagree that any of these things can’t reduce you by 3mpg. That is especially so for the thermostat and coolant temp sensor.

Maybe getting the truck scanned for any possible codes would be the ideal first step.
Autozone or whomever will do this for you free.

Winter gas can also cause lower fuel mileage.

What is winter gas?