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Toyota Corolla 2000 with Check Engine Light reading PO 171, PO 172, PO 420 ! HELP

My Toyota Corolla 2000, 123,000 miles, Mass license plate, still getting 34 miles per gallon, has had check engine light on and off for a year.
Failed inspection this June.

First garage in Somerville, without doing more than getting the three readings above, said catalytic converter was needed $350 plus labor.
I was unable to do the work then and returned to NYC and job. Took to another mechanic recently to get fixed (as I wasn’t returning to Mass for a while)
and got only PO 171. I mentioned the other readings; they kind of scoffed it off.
Trying to save me money (I guess), they cleaned the MAF filter and fuel ejectors for $200.
Looked like it was fixed. A week later check engine light came on again and when I brought in found the PO 420 reading.
They did not tell me if other computer read-outs were coming up as well.

Now they want to leave the oxygen sensor in front but change the one behind. Put in a new
catalytic converter $539 {they had quoted me $238 but it wasn’t an OEM one I guess and they said they wouldn’t warrenty anything but this expensive one), charge me
$137 for the oxygen sensor
190 labor
plus tax
for a total of a whooping $946!

I would do this but reading on web that the mechanic should have diagnosed the catalyst efficiency with a pyrometer and verified with a gas analyzer. I don’t know if he did (certainly not at the first garage) and am worried that i am going to pay close to $1000 [after paying over $700 a week before: $200 MAF filter clean; $250 tune up and parts!; $230 ac checked and re-filled but leak not found plus tax].

I am afraid they will do this very expensive job and the check engine light will come on and htey will say o its something else.
how can i be sure? also I was reading for a catalytic converter to go bad, something has caused it…what would that be? how to find out. Do they need to do the diagnosis with gas analyzer etc?

Please help before i spend a huge amount for an old car that is running well, not losing gas mileage, but perhaps is a bit slower on power [ a sign i read that could be the cat].

Help! I am trusting a very expensive garage and wonder if i should.
thank you if you can help asap.

First off, the fees you are being quoted seem very reasonable for the work mentioned. Installing a new OEM cat usually costs more than that. I wouldn’t say this is a particularly expensive shop. Whether that’s a good thing or a bad thing, hard to tell on the internet. Did someone you know recommend this shop to you?

Diagnosis of a bad cat usually involves taking some emissions measurements and doing a back pressure test. Often only the back pressure test is done though, as the car’s computer does the emissions measurements using the pre- and post-cat O2 sensors and is now reporting something is amiss. Of course it could be the O2 sensors are the problem, not the cat. But O2 sensors are pretty tough. Usually good for more than 123K.

The P0171 is a mixture too lean code. The P0420 is for the cat efficiency. It’s quite possible the lean operation has damaged the cat, or at least caused the cat code to appear. So it’s important to address the lean problem first. It may be that after the lean operation problem is solved, the cat code will go away. If you Google P0171 it will give you some idea what the possible causes are for lean operation. Your mechanic just needs to go through those one by one until the problem is resolved, then see if that causes the cat code to go away.

If you simply must get the car through the inspection process before some fines start accumulating, it may prove less expensive just to install a new cat. But be sure that the lean operation business is fixed too, otherwise you’ll be looking at another new cat for the next inspection.

Here’s something to think about, and it’s essentially what George is saying.

P0171 means lean. Because of the lean mixture, the upstream sensor won’t be switching as much.
P0420 is generated when the PCM sees that the downstream sensor is switching nearly as fast as the upstream sensor

See what I’m getting at?

Before even condemning the cat, I’d determine what is causing P0171

Leaky intake manifold gasket.

An evap/smoke machine should find a leaky intake gasket in just a few minutes

But there are also other causes for a lean condition

I’ve worked on vehicles in which low fuel pressure caused P0171

Amazingly enough, some of them started up pretty well, had a nice idle, and seemed to have decent power.

Antoinette: Is your CEL on NOW? If so, disconnect your negative battery cable for a minute or two, re-connect it, the CEL should now be off…Drive the car normally and see how long it stays off…If it stays off for a day or two, go have it re-tested and it will probably pass…Drive on…

If it comes back on in less than a day, replace that #2 oxygen sensor (the rear one) and try that first, before you replace the CAT…

Thanks all!
The shop is one I have used before. I am glad to hear you confirm that they are reasonable.

First thing they did when Lean PO 171 came on was “smoke engine to locate leak, clean mass air flow sensor. purge and flush fuel injectors. clear codes and reset system”. I drove for a week and check engine light came back on. Very disappointing. I believe they were trying to save me money.

Now they get the cat indicator. Please note though as I noted previously, the month before at different garage in Mass (i live in NY, work in Mass), PO 171, 170 and cat came on: all three originally.

I don’t know if they have checked the gasket. I will ask. and they are planning to replace the rear oxygen sensor and the cat. I don’t know if they have diagnosed the catalyst efficiency with a pyrometer and verified with a gas analyzer. I will ask. Should I get both done if so: oxygen sensor and cat?

Also what could have caused this? in 09/10 it was parked for a year with battery out. I had it tuned up after and try to take good care of it.

Thank you again for suggestions. Very very helpful to try to understand this. Alot of money for an old car that runs well.
Antoinette vs

Was wrong in that i had not noted previously that all three codes came on a month ago.
Will wait to hear from you all one more time and then I will decide finally what to do.
Thank you!

spoke to the garage. they said no leaky gasket. as noted, they “smoked” the engine originally.
They are a honest garage i believe increasingly. However they don’t have a pyrometer or gas analyzer. They said they had a scanner and saw everything working but at a too low temperature I believe? Would it work to just have the rear oxygen sensor fixed perhaps?

If you’re going to keep this car, put the $1,000 in it. You should plan on having a $1,000 emergency fund for this car at all times as you are likely to spend that much per year on any car this old. BTW, that is a cheap way to own a car; you just have to expect breakdowns.