lower milage and O2 cross count

Odometer reading?

Have the iridium spark plugs been replaced?

 

I would suggest that you check the following:

1. Wheel alignment. Front should have zero toe-in and the rear should be within toe-in spec, ideally at center of range.

2. Parking brake drag - raise up the rear axle, release the parking brake, and rotate each rear wheel to see if the brake is dragging.

3. What tires have been installed during your ownership of the car, has the mpg decline coincided with installation of new tires; and what tire pressure are you maintaining? I suggest that you use 44/42 psi (front/rear) consistent with that not exceeding the max cold air pressure label on the tires' sidewalls.

4. I would expect the A/F ratio sensor to show rapid voltage fluctuations while the downstream oxygen sensor will show more constant voltage readings. Given your low odometer reading, I would not suspect that a sensor is bad lacking engine ECU DTC pointing to that.

5. Have your commuting habits changed over the last several years?

6. Has the fuel in your area changed over time, such as by having a greater proportion of ethanol (which has lower energy content), etc.

 

Before you measure A/F sensor and oxygen sensor voltages, the drivetrain should be fully warmed up so that the sensors are working. If you drove 15 minutes at highway speed that would be OK but if you drove 15 minutes at 25 mph that may not be sufficient.

O2S12 is the downstream oxygen sensor so its voltage reading should be more constant.

After you installed the BF Goodrich tires, you didn't notice a mpg drop at that particular time?

When you say "front battery" I assume you mean the 12V battery which happens to be located in the hatch area, near the right rear corner of the car. That battery is now ~5 years old if it was replaced in 2006. A weak 12V battery may impair mpg so you may want to consider replacing it now.

 

now hold on there pardner!

 

The above seems somewhat unlikely esp. given http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=177 (if you replaced the worn stock tires w/identical model new ones).

What are the make model and size of the new/current tires? (It's likely they're higher in rolling resistance than the stock ones.) re you manually determining your mileage after each tank? Also, are you manually resetting your MFD after each tank?

As for the latter, neither the 12 volt nor big NiMH HV battery are under the hood nor next to the engine.

 

big battery next to the engine? what battery? :huh:

There are two - one is in the back, under/behind the rear seat, and one is in the WAY back, in the rear of the hatch area. The rear seat battery is the hybrid battery, and the hatch battery is the 12V.

 

if your mpg's dropped after they replaced the hybrid battery, they probably did something wrong or the battery was not upto snuff. you may now be seeing further effects from that.

 

Why was it replaced? Seems very strange to need replacement prior to 68K miles.

You're not giving us a lot to go on. You haven't answered any of my questions.

Although I'm not the repair expert, if I had some supposed mileage drop, I don't think I'd start fishing by looking at O2 sensors at 68K miles.

There's http://priuschat.com/forums/gen-ii-...-answer-these-questions-esp-if-youre-new.html, but we need to start at some basics such as the questions I asked earlier. I don't know the MFD mileage reset behavior (and if you're even using that to determine mileage). For all we know, you haven't been resetting it (and the car doesn't reset itself after each fillup) and you've just been seeing the average fall over a long period of time when in fact there could've been a sharp drop long ago, such as at tire replacement time.

 

That seems pretty strange Rick. You're saying that your traction battery went out at under two years (and presumably under about 25000 miles given your current odo reading).

It's also strange from the perspective of your MPG going down after the battery was changed. Normally the MPG would go down when the traction battery was going bad, and then improved significantly after the battery was changed.

Can you give any more details about how your car was running when the traction battery "wouldn't charge". Did the car completely fail (and have to be towed to the dealer for example) or was it more like the state of charge just being persistently low when this happened.