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Discussion Starter #1 (Edited)
I drive 2005 Grand Caravan 3.3L with 55K miles and its getting 13.7 mpg in city (down from 18 mpg). Vehicle well maintained and in excellent shape overall.

- Had intermittent CEL (O2 Sensor bank 1 sensor 2) and replaced downstream O2 sensor. CEL stopped but mileage has not improved. I have not replaced upstream O2 sensor because no CEL code indicating failing.
- There are no secondary indications of problems - engine idles, accelerates, cruises smooth. Have not checked for binding brakes but do not think that is cause. I have eliminated all the other "usual" causes for poor mileage (fuel source, driver, roads, etc).
Does anyone have ideas on what to do next to troubleshoot cause of very poor mileage? Can a weak upstream O2 sensor be the problem (even if no CEL/fault code)? Would it be good idea to replace it too?

Thanks in advance.
 

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Dragging brakes would be one thought. After driving for awhile, see if any of the wheels are too hot to touch or at least noticeably hotter than the others.
FYI, changing the downstream sensor does nothing for engine operation. It only monitors what the cat is doing, nothing more. It's the upstream sensor that controls the engine.
 

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Discussion Starter #3 (Edited)
Thanks for the note mfahey.

I have checked for signs of dragging brakes... no indications of it at all.

I may bite the bullet and change out upstream O2 sensor with one made by NTK (part # NTK23566). If so, will be a hassle because there is a TSB out which says must do a PCM re-flash to make sure the system calibrated and performs properly. Apparently it has to do with a mid-year change made by Dodge.

So it ultimately means having to go to the dealer... for PCM flash. Hopefully it is covered by Dodge as "mandatory" maintenance. After all that will do mileage checks to see if any improvement.

I will continue to post updates to this saga so everyone can learn from the experience.
 

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Discussion Starter #4 (Edited)
More on the TSB which says mandatory to do re-flash when replacing upstream O2 sensor:

According to the NTKO2-TSB-0414-EN after mid year change (Jul 2004) Grand Caravan has XZZ option code and uses sensor OE# 56029085AA only. It also required PCM software change (because O2 sensor specs went from 2-4 ohm to 7-10 ohm).

Apparently there are discrepancies in some vehicles on the road, i.e. software and sensor do not match. So only way to be sure is to put the XZZ option sensor in and flash the PCM with "known good" software.

I sincerely hope I do not have a mismatch in my van causing the poor mileage. In any event I will be taking action to certify correct O2 sensor /software combination are in my van. Stay tuned.
 
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