Decreasing MPG & Check engine light

The Prius is like any other car. The mileage is best in the summer and worst during winter. There are a number of things that factor in, so I would not fret over the MPG dropping the small amount you are seeing.

Concerning the CEL, I would have the codes read. With todays cars a CEL can mean a whole of things. It can be minor, or major. Without knowing the actual code, it is guesswork.

There are options concerning reading the codes. Your best bet but also the most expensive it to go to the dealer and ask them to pull the codes for you. They will charge you a one hour diagnostic fee for that. You can go to AutoZone or some where similar and ask for the codes to be pulled. Their scanner will pick up some of the codes, but not all of them. Autozone is the cheapest route, but the information may be incomplete. If you know someone who has a laptop with Techstream software on it, they can do what the dealer will do, thereby saving the diagnostic charge.

Bottom line: Have the codes pulled. Once you have the codes, post them back here and someone can give you an idea of what needs to be done before you have the work completed.

If you have money to burn, trust the dealer. If not, then try alternative sources. The dealers tend to charge a lot, and some don't do their jobs correctly. Most do, but some do not.

Remember that the CEL could mean a lot of things. It is not just a oxygen sensor like it used to be ten or fifteen years ago. Best of luck to you and I hope this information helped you.

Finally, Welcome to Prius Chat!

 

The tech defined the code right, P0138 points to the rear O2 sensor and or faulty wiring. Some who have had this problem have ended up changing both the rear sensor and the forward air fuel sensor before the cat converter.

Any damage to the cat converter that you know of? Do you have a trusted mechanic other than a Toyota dealer? If you have a good trusted mechanic, I would order a new rear O2 sensor and have it installed, then clear the code and see what happens. The dealers tend to charge too much.

P0138 technical jargon: P0138 Toyota Oxygen Sensor Circuit High Voltage Bank 1 Sensor 2 OBDII Engine Light Trouble Code | Engine-Codes.com

If Patrick Wong posts on this string, follow his tips. He is very knowledgeable concerning the Prius.

I am no expert when it comes to emission controls, so at this point I am relying on information found here on Prius Chat. I hope this helps you.

 

Driven through any deep puddles of late?

 

See my comments below. Although it is normal to see declining mpg in cold weather, your engine may have a problem, but the air/fuel ratio sensor which is upstream of the catalytic converter is a strong suspect given the decline in mpg associated with the CEL coming on.

You could order the sensor online but make sure you have lined up someone who will install the replacement sensor. (The dealer's service department may not, if you provide the part.) Also, buy the correct Toyota-branded sensor, not some aftermarket universal part.

Typically, a rear oxygen sensor problem will not result in reduced mpg.

Here are two examples where the actual culprit was the air/fuel ratio sensor which is upstream from the catalytic converter:
Another CEL Thread!!! P0138 and P2195 | PriusChat
Prius Gen 2 P0136 P0138 Dtcs - Hybrid / Toyota Prius - Toyota Owners Club - Toyota Forum

 

One way the mileage will start to tank is if the 12 volt battery is starti

Another way the mileage can tank is if the 12 volt battery in the trunk is starting to go bad. If you bought the car off a car lot I can almost guarantee you they allowed the 12 volt to go dead and had to jump it. This kills that little agm battery.

Please do this test its very simple. We especially want the second number the load number:

Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

Every fall we got alot of these posts. Winter kills a so-so battery. Right before the battery leaves you stranded it throws a big red exclamation point on the dash at start-up. Only there for a second like all the other lights that come on for bulb check.

Millions of posts about this little battery on this site. If I bought a used Gen 2 off a car lot I would replace that battery. You do not want to be stranded with this car.

 

I would take it back to the place that did the repairs and ask why. If the same codes are appearing, it sounds to me like the car was not repaired properly, or being misdiagnosed.

 

Are you referring to the oxygen sensor which is downstream from the catalytic converter, or the air/fuel ratio sensor which is upstream from the cc?

Yes, raise up the car so you can get under it, and look to see if the part is shiny or not. Take photos if you have any concerns.

Did you see my post #8 where I indicated that the air/fuel ratio sensor is the likely culprit here?

 

Which battery? That was just the small 12V one I assume.

It's pretty easy to test the battery yourself to confirm whether or not it really needs replacing. It's a bit late now, but it definitely would have been better to test that for yourself to avoid having the dealer potentially rip you off.


Sounds like they're having a bit of crap shot there. Two repairs down and they still haven't got it. I suspect Patrick might be right about it really being the fuel/air sensor.

 

Bank 1, Sensor 2 is the downstream oxygen sensor. Upstream or downstream is relative to the catalytic converter. Bank 1, Sensor 2 is at the catalytic converter output, hence is downstream.

If you would pls. read the cited posts in my #8 message, you will see some stories that indicate a failed air/fuel sensor (which is Bank 1, Sensor 1) will potentially trigger the DTC that your car logged.

 

Agree 100% with the above statement. I also mentioned that in post #5. The code your Prius is giving can be troublesome at times. That is why I suggested you take it back to the place that did the repair and have them do the job correctly. If you take it somewhere else, it will simply cost you more money and they may try to do the same thing the first repair shop did.