O2 sensor question

[gw33gp]

I have not replaced my O2 sensors in over 150K miles. I am noticing fuel mileage starting to drop, but my Ranger is still running great. I thought I would try replacing the O2 sensors since they are overdue anyway.

A question is, would worn upstream O2 sensors have the most affect on fuel mileage? I am thinking the downstream sensor is more related to emissions check more so that the upstream O2 sensors.

The one downstream O2 sensor for a manual transmission cost more than both of the upstream O2 sensors together. I thought I would start with the upstream O2 sensors first to see if that makes any improvement in fuel mileage, or should I just bite the bullet and buy all three?

 

[Dirtman]

The downstream sensor lasts longer and is basically just there to monitor whether or not the catalytic converter is functioning.

 

[RonD]

Yes, upstream effects mileage the most, as they run out of chemicals they show leaner voltage so computer runs the engine Richer than it needs to so MPG goes down

All Ranger(not 2019-2021) O2s are the same kind, narrow band, the difference in cost and part numbers are the length of the attached wire and connector
So shouldn't be that big of a price difference between a 6" lead and a 24" lead

Downstream O2 sees less Oxygen so tends to last longer, but still looking at 12 year life span
Downstream can effect MPG a bit because Cat converters need unburned fuel to keep them hot, so failing downstream O2 will cause computer to add extra fuel more often to keep Cats hot even though they were working fine
If the Cat is failing that can lower MPG because downstream O2 is showing that its failing but you would see a code indication that

There are no O2 sensor codes until you get up to a 15% error/difference, so your MPG can go down by 14% without any codes, because computer doesn't know there is an issue, there is no MPG sensor, lol

 

[gw33gp]

Thanks guys. That confirms my thoughts but I had no facts to back it up. Now I do, I trust these are facts.

I will be ordering two new upstream O2 sensors.