The more I replace, the worse mpg gets. WTH?

Well if you think about it a burning house can usually stay standing for a while after the smoke detector starts beeping. That doesn't always mean it's a great idea to keep living there.

Somebody needs to do real diagnostic work to figure out if this problem is centered in the catalyst or if the problem is further upstream. If the engine isn't running correctly it could be creating a legitimate catalyst efficiency problem.

Given that you've done ignition and airway work recently it may be worth revisiting those just to make certain. Both of those departments can contribute to misfires, misfires can foul the cat and it's off to the races from there.

 

Have you checked your brakes recently or are they hot after a drive?

 

Spend about 350 bucks and buy a complete new cat assembly from the exhaust manifold flange to the flange at the rear wheels. Install it and see what happens. Take the original one and sell the 2 Toyota cats on ebay for 850-900 bucks. You make 500 bucks and have a new cat.

 

If you're running elevated tire pressures on rough roads, that might be contributing to wheel bearing failure. Toyota recommendation is 35/33 psi (front/rear).

How many miles on the car btw?

 

How many miles on her, and is that Castroville in California?

Check those plugs again, could be fakes if not purchased from a local retailer.

What does your average commute/trip look like in time/distance, etc.

 

^ yup, lots of counterfeit plugs on ebay & amazon lately. Probably others too.

 

Look before you leap.

P0420 ≠ bad catalyst.

It might be a bad catalyst, but really that code means diagnostics are required so you can work out whether it's a leaky injector, undetected oil burn, flaky throttle positioning motor, bad wire on a sensor etc etc etc.

I don't have the specific workup for that code on your car, but maybe somebody else has it handy.

 

None of the application specific or direct fit options can be legally shipped or installed within CA.

You might find this post/thread helpful. Installing a non CARB compliant catalytic converter could cause you to fail the CA smog (visual) test.

 

Sounds like what happened to me with my 06 Gray 297,000. The cat was clogging up. It finally failed and the internals broke apart. Ran fine once I took it off and emptied out the melted ceramics !!!! Yes it still threw a code, but the mileage went back to normal.

 

First off, your 39-42 mpg was a little low at the start. I consistently got around 45 mpg with my gen 2. When I purchased it back in 2009, I had heard that the Prius was notorious for wheel alignment issues and you had to keep an eye it, but I never had a problem with this. If you didn't purchase your gen 2 new, you may have inherited a lemon.

 

...or maybe a bad cat, as mentioned above... ...I hadn't thought of that. Did you get a chance to check that out? I'd hate to see you drop the dollars, only to find out that the cat wasn't the problem.

Before you take it out to inspect tho, I would look into having your cat cleaned by someone who does that kind of work, or turning it in to getting a cleaned/overhauled replacement. There are a number of threads here regarding this subj. The problem, of course, is having to wait for it. Or perhaps, if you are brave, you could try cleaning it yourself (see highlighted link to PC article).

 

Belay my last. I was confusing a contaminated catalytic converter with clogged egr issues. Completely different problem. I wouldn't think that a contaminated converter could cause the low MPG numbers you are experiencing.

If you really need to replace your cat tho, you still might want to look into having you catalytic converter cleaned instead of replaced.

 

Are you sure you're state of charge is usually in the 40 to 50% range??

In my gen 1 Gen 2 and gen 3 if state of charge dropped in the low fifties it was doing everything possible to bring up the state of charge.

all three Pri that I've had the car was usually between 58 to 64% state of charge very very rarely have I ever seen at 55% or below..