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Replace a catalytic converter? To pass an emissions inspection?

I ask is it really necessary to replace a catalytic converter (at ~ $1200 and up) after the check engine came on, and one of two mechanics said a bypass valve was plugging up, and the solution is to replace the “second” catalytic converter.
This car, a Prius, has twice failed a State inspection of auto emissions. The diagnostic codes were (at first) P 171 and P1436; but now only P1436.
The first mechanic cleaned the “mass air flow sensor” and cleared the check-engine light. However that light came on again during the second inspection.
Is there a simple alternative to replacing the catalytic converter?

If it must be replaced, can I obtain one in good condition from a junkyard or some other market?

Toyota, Prius.

Oops ! You Forgot The Model-Year !
What Model-Year Is It ?

where i live in NJ can’t sell used cats. there are other options aftermarket . what year is the car?

I had a funky choke on old car and it damaged cat. This was after 6-7 yrs of owning car. Was seldom driven at end. Annual inspection said exhaust was 219 ppm and failure level was 220. Whew. Mileage was 30mpg or so during life but at end it was 20 or so avg? I am almost sure cat was plugged but we decided to sell car. A new cat would easily have fixed mpg issue. Paid for repair too.

The model year of the Toyota Prius is 2001. (I had asked if it’s necessary to replace the catalytic converter.)
Since I posted the question the first time, I’ve talked with the second mechanic, who is certified in Hybrid Service Technology, and Toyota/Lexus engine performance.
The faulty part is a “hydrocarbon absorber valve,” which is part of the front exhaust pipe subassembly. The part was updated by Toyota in 2002, so the problem was mainly in cars from 2002 or earlier.
This part cannot be replaced except by replacing the whole “front exhaust pipe subassembly” which includes a muffler, catalytic converter and more . . . thus $1192.

I asked the mechanic about an alternative solution.  Today, he cleaned this hydrocarbon valve as best he could, with penetrating oil, and suggested:
- I drive the car about 100 miles to reset the diagnostic codes;
- try the "country fix":  to accelerate the car flat-out a few times, in the hope of blowing carbon deposits out of the exhaust system;
- maybe add fuel-injector cleaner (an additive) to the gas in the gas tank; 
- take it one more time to the State emisssions test. 

Stay tuned.

I will see about getting a used “front exhaust pipe subassembly.”

Your state probably has a limit on how much you must spend to pass an emissions test…When you reach that spending plateau, you are granted a waiver whether it passes or not…How many miles on the Prius?

Toyota Supports ASE Certification Page 1 of 1
’01 – ’02 Prius
Technical Service
August 2, 2002
Certain 2001 and 2002 model year Prius vehicles that are operated in areas where road
salt is commonly used may experience a M.I.L. “ON” condition with Diagnostic Trouble
Code P1436 stored in the Engine Control Module (ECM). Use the following procedure to
inspect the HCAC bypass valve for smooth operation and replace the Front Exhaust Pipe
Assembly if any shaft binding is evident.

2001 – 2002 model year Prius vehicles produced BEFORE the VIN shown below.
Prius JT2BK12U#20051999
17410–21260 17410–21261 Front Exhaust Pipe Assembly 1
Standard Metric Socket Set & Hand Tools As Required
Refer to the Engine Diagnostic section in the applicable Prius Repair Manual, Volume 1.

@Caddyman, you have to spend the money first. In my state, that starts at $700.

@All4Hybrids, given the TSB is dated 2002, and the fact that you have a 12 year old hybrid, I’d bite the bullet and spend the $1,192. If you can find a used ‘front exhaust pipe assembly’, chances are it will not be in much better shape than the one you have, and will still cost you several hundred. You’ll be chasing your tail with this repair. Unless the goal is to sell or trade the car before next year’s inspection.

Also, don’t bother taking the car in for inspection if the CEL is on. Instant failure.

We own a 2011 Prius and live in California. We were told at a SMOG inspection station that Hybrids are exempt in CA.

@ProVega350 I presume OP’s Prius is not exempt in their own state.

Is there any update on how this repair went? Did you replace the entire front exhaust assembly? I have a 2002 Prius and the “second” catalytic converter is rotted out. There is no code, just a lot of leaks. Can I just take it to a welding shop, have them cut out the bad cat and weld in an aftermarket cat? Since there is the actuator and special hydrocarbon capture function on the first gen Prius’ second cat, I am worried that if I just remove all of that and replace it with a generic aftermarket one, it will throw a code. The welding shop guy says that he’d be willing to do it, but I’d be risking it throwing a code due to changes in the back pressure. Seems to me, though, that I could just cut out the bad cat with a hack saw and weld in an aftermarket cat, rather than buy a $1000 whole new assembly. Or where could I find a used “second” cat specific to this model? Thanks!