Prius is extremely slow/poor mpgs now

Sounds like you have a bad hybrid HV battery I don't know about any codes or any of that but sounds like something is robbing you of power and I'm not sure that that's necessarily in your engine or the ice portion of it usually this kind of shows up in the high voltage electrical end of things The motor generators in the transmission that sort of thing but you never know I run the same kind of car I have an '05 and a '09 I had quite a bit of this type of thing going on when my battery was 13 years old and falling on its face The minute that was changed was back to pretty normal I've never seen 5055 miles to the gallon on any kind of regular basis and I've put over 130,000 mi on generation 2 vehicle

 

You need to get the car to a good, professional, certified mechanic for a proper diagnostic.

I would start with replacing the O2 sensor, with an OEM sensor - NOT an Amazon or Ebay one. That sensor determines air/fuel mixture and is sounds like the car is running very lean.

Sputtering and chugging on cold starts are signs of a head gasket issue.

Struggles to accelerate: probably only running on 3 cylinders. If you can't diagnose something as simple as that - you need to take it to a mechanic!!!!

Sorry, That's all I can get out of my Crystal Ball - reminder; this is an internet diagnostic. Alot of shade-tree mechanics likes to just throw parts at problems; but I usually see them introducing even more problems with bad, counterfeit parts, breaking stripping and bending electrical connectors along the way.

 

It could be a good sensor delivering bad news for some other reason. What are the actual diagnostic codes active at this time?

 

Up stream before the cat? Isn't that the one that connects under the floor in the passenger footwall close to the center hump?

 

If it has the original hybrid battery there you go. Its 18 years old.

You never mentioned the battery consumption behavior on the mfd the screen above the radio. Does it tank to 1 on no blue bars real fast at a light or in a drive through? And takes blocks to put back some blue bars?

Do you ever see purple battery bars?

The bulk of all power comes from the hybrid battery. Without its 100% participation the car is now a 90 horsepower go kart and your basically flooring the car to get it to go. This is stage 1 of battery failure.

And your flogging the engine to prop up the loss of battery power that's hard on a high mileage old engine so keep an eye on the engine oil level.

Battery failure on a G2 is extremely common. Probably a thousand posts about it now on the G2 forum.

 

If you are having problems while the car is at speed, it is more likely an engine 'making power' issue, not a HV battery issue.
How does the car respond when accelerating from a stop? When under 42 mph, are you able to go battery only?
Which Amazon catalytic converter did you purchase?
Which O2 sensor did you install? Brand and model?
Rock Auto has the Denso O2 sensors for about 50-60 bucks. I've had great success with those.

Sometimes we have to remember the hybrid operation. In simple terms:
Atkinson cycle engines are weak on the bottom end, this is where the HV battery does the majority of it's 'thing'.
They are very efficient when at rpm. That's where they do their thing.
That doesn't cover all possibilities, but just in general.

 

the diesel probably has a lot more torque than the atkinson

 

So what codes are active?

 

go to your car and go into the service menu on the screen.. engine off, ignition on, hold brake pedal, info button and switch headlights on and off 3 times.. turn off all electronics, fan, radio, lights door closed etc.

report the voltage.. then turn on lights, and fan on high and report voltage.

turn engine on, ready position, and repeat above..

 

This is not complicated. The battery may be old, but it's fuctioning normally, especially with the low speed being normal.
When the car is at high speed, the engine is not producing enough power and it's causing the battery to drain because the battery is trying to assist in a situation where it shouldn't be.

This is not the first time we've been through this!!

You need to find out why the engine is not developing power. It takes air, fuel, spark and exhaust. One of them is missing!!

Sometimes you need to fix the obvious first. No upstream or downstream? Why would it be surprising if the cat was damaged? Pull it out and look at it, if needed, ram the catalyst out and try the car again with the free flow exhaust. If problem fixed, then at least you know..

 

Go on Amazon buy a Carista Bluetooth Dongle there like $40.

Then get the DR Prius free phone app. You need that bluetooth Carista so your phone can talk via BT to the obd connector the carista plugs into. Dr Prius will pull hybrid codes and has a hybrid battery load test feature.

In the meantime lets see how tired that battery is:

Find a nice long flat stretch of highway. Need about 3 miles no lights.Shut off all accessory's no AC etc roll up the windows. Get the car up to 65 miles an hour. Turn on Cruise Control. Reset the mpg meter on the mfd, Clear it.

Don't touch anything just let it run at 65 for about 3 miles. Make sure no hills.

At the end of the 3 miles whats the mpg meter say on the mfd?

I'm betting its 40-45 mpg. Stage 1.

A perfect G2 hybrid battery will be 62-65 mpgs. That's what my 07 with a brand new battery pulled. It also pulled the 42-45 pre new battery.

Good Luck.

 

I'm curious what the battery test was that resulted in a 696 value. Maybe I'm having an elder moment, but what does the 696 represent?

 

must have been one of those eLoN MuSk testers, 420 means good, 696 means good, 42069 means GOOD....

in all seriousness, 696 isn't even possible unless they are running some crazy lithium iron battery.. brand new optima yellow tops are 450CCA and I just put one in 2 hours ago and can verify...

 

A $115 "cat" from Amazon will be a formed pipe and nothing more. There just isn't any other way to do the catalytic chemical reaction without a few hundred bucks worth of precious metals included in the auto part. So what you've got is a pipe that keeps the exhaust manifold connected to the resonator and downstream piping. No pollution reduction possible.

The good news is that such a pipe is not likely to clog up the way a real catalyst would.

The engine ECU is very dependent on the AFR data from the upstream sensor. I do not know if any process monitors the downstream for any data that would affect power output. (That does occur in some cars, I just don't know if the 2nd generation Prius is one of them.)

You can always leave extra wires dangling to discourage repeat theft, but you should have your real ones doing their jobs to have any chance at normal operation.