New catalytic converter, lost 10 mpg

did you change the oxygen sensors?

 

is the flow going in the correct direction??

 

Why was the cat replaced?

 

not sure about direction, just a thought.
I was told to change the O2 sensors with the cat, of course it might just be a way to get more money out of me from the muffler shop.

 

bad week for repaired prius, my battery rebuild failed,took out for a fast coffee run this morning and its got the same problem as before but the red triangle has not come back, YET..shit happens with old cars..

 

a P0420 is most commonly the downstream o2 sensor , with an upsell of a new cat. Sometimes the cat is needed and can be confirmed with a flow test. Did the P0420 come and go or remain constant?

 

cat?...looks like a dog to me...
do you think they cut off 12v power during the repair? Sometimes seems to need a tank or so to reset after that. Or maybe ran it low.

 

If Cat was originally poisoned by the engine (ie high miles oil-consuming engine, tune up-starved, oil/gas additives etc), those conditions will still deliver poor MPG, and will eventually poison new Cat too if not corrected. An exhaust restriction inadvertently welded into system will show as loss of power at higher speed demands along with poor MPG. Sustained slower highway speeds, however, should still result in good/normal MPG with slight exhaust system restriction (1-1/2" pipe 86" long is not much for a 1500cc engine. Personally had deep rock ding that may have ruined my Cat.
Air/Fuel (front A/F O2) sensor should definitely be replaced when doing Cat with OEM Toyota A/F sensor (1 hour or so job for experience dealer-tech via windshield wiper assy/rain pan access, NOT the excessive 4hrs and $700 as some stealer/dealers claim, $400 dealer max, DIY=$150 been there did one). But keep in mind, there are many many reasons for poor MPG as we all know, even low tire pressure and/or wheel alignment.

Really registered and came to PC, for first time however, to post sneaky little Smart Key advisory and/or fix I found, just couldn't resist on the Cat/MPG

 

Some catalyst poisoning is reversible, with operation at high load being all you need to restore the cat.

I have an 82 pickup with ~400k miles, a 92 Camry with ~200k+ miles, and a 05 Prius with ~200k+ miles, all passing smog on the original catalytic converters. I'm not sure if you still have the old cat, but it might be worth hanging on to.

 

Some info for OP .

OBD2 Error code P0420 - Catalyst System Efficiency Below Threshold possible CHEAP FIX for any car.

 

FYI, while P0420 *can* be a sensor issue, such cases seem to be exceedingly rare according to every tech I've come across around these parts. I suppose it doesn't hurt to spend $100 or so, just in case that's the fix that's needed, but far more often than not, it'll end up being the catalytic converter.