I've been working my way through my list of maintenance items for my 04 Discovery and next on the list of my to do's is to change the 02 sensors. Why? Well I reasoned that they haven't been changed out and the Atlantic British website hawking them says every 60K miles or so is a good interval. I figured that replacing proactively might lead to better fuel economy...?

I was talking with my neighbor, who manages a Mercedes service department, and said don't bother. He said he believes the on board computer monitors the performance and if it deviates from the accepted performance range, it would throw a check engine light. As long as the light is not coming on, spending the money to change them out is not necessary.

Sounds reasonable! So, I guess my question is...leave enough alone with the 02 sensors and move to my next item on my list? Or, short of a CEL, is there any real gain in swapiing these out.

 

its roughly $150 for the pair and 10min to swap. do it now cause in all likelihood you will be doing it soon. chances are one will crap out soon. its the norm on these trucks. some go longer some dont. my 04 tossed a O2 code at 57k miles. so there is no set time, but its easy preventative maintenance

 

You only need to change out the fronts.
Yes changing them will help your MPG.
The only time they will throw a code is if they are so far out of whack the ECU cannot compensate for them or the heater in them fails.
Think of it this way, you're only bleeding alittle bit so why bother with a band aid? Why not wait until you are bleeding so bad you feel light headed before you go to the hospital?

Do this for your neighbor, keep track of your MPG for the next 2 weeks.
In the mean time order your new O2's.
Then install the new O2's and keep track of your MPG for another 2 weeks.
Show him the difference.

 

X2 what Spike said. The rear sensors only monitor the catalytic converter efficiency. Unless you're super conscious of your emissions, only change them if they throw a code. The fronts directly effect fuel economy. As they age, their switching time gets slower and slower, and they will not set a code until they get so far out of range. But, the slower switching times will cause increased fuel usage. I changed my fronts out last year at ~100k (no codes) and got about 1-2 MPG on average better after the change.

 

Thanks, guys. Just the front two then. I track my fuel mileage every fill up, so it will be easy to see any resultant gain fuel mileage. Next weekend is a new rotoflex for the rear driveshaft and changing the transmission filter and fluid. I'll get to the front 02 sensors after that.

 

What do you average right now for MPG?

 

Who, Baker or I?

 

Baker.

 

Over the last 1000 miles (977 actuallly), I have averaged 14.02 mpg The best average at fill up over the 4 weeks (977 miles) has been 14.41 mpg, the lowest was 13.83.

I use 91 octane Chevron Supreme and probably 90% of my driving is on the freeway -- but this is Silicon Valley freeway commute-- some stop and go, small stetches at 60 MPH. I cover 30 miles in 50 minutes on the freeway, so averaging about 36 MPH.

Earlier this spring, I drove up to Tahoe -- a longer stretch of freeway driving at 65 MPH. In that scenario, I was averaging just over 16 MPG.

 

That sounds a bit low for highway driving.

Assuming the rest of your service is up to date and your tire pressures are correct (and you don't have a roof rack or other massive drag-inducing mod bolted to the outside of the truck) front O2s might pay for themselves.