P0420 - 2002 Prius 285000 miles

Our 2002 Prius has 285000 miles on it and has had the check engine light on for a couple months. I had the shop pull the code and it's P0420, which indicates a catalyst system / catalytic converter problem. They gave me an estimate of $2123 to replace.

I'm having a different shop more geared towards mufflers/exhaust look at it next week. $2123 seems extreme and I have no idea what all the first shop is including in that, as far as oxygen sensors etc.

The engine has made a subtle knocking sound for over a year now, which seemed to coincide with the first shop replacing the ignition coils. I've just lived with it, but thought I'd mention it in case it's related. The check engine light / P0420 is much more recent. I've also noted over the past 6 months or so that the mpg has gone down from the high 40s (winter) - low 50s (summer) to consistently around 46, i.e. roughly a 10% drop. At first I thought this was due to new tires, but I'd never seen that kind of drop previously having gone through at least 5 different tire brands (currently Falken Pro Touring A/S).

I'm not a DIY guy so I'm wondering what I could be getting into with this P0420 thing. The car still drives like a champ otherwise, but it's 20 years old, the dreaded big red triangle has gone off several times over the past year or two indicating a HV leak, and I'm not sure if we're at the "let it die" phase or not. I just put $650 into replacing one front wheel bearing (the mpg hasn't improved after that) and formerly the most reliable vehicle I've ever owned is becoming a money pit.

Any insight appreciated. Thanks.

 

The above quote is from a local shop, not the dealer. I called the dealer where I purchased to get an estimate yesterday and they have not called back yet. The muffler/exhaust shop said it might just be an O2 sensor or something else. But after 20 years I'd guess it's the cat converter, over $2000 just seems extreme. Used/new cars these days are so expensive it could be worth it, but I'm also looking at having to replace the HV battery soon (at least three "HV leak" codes / red triangles in the past year) for another $3000... Thanks for any opinions.

 

Sorry for disappearing. I'm not receiving alerts when someone posts here.

As of yesterday: The second shop started with replacing the upstream O2 sensor. They seemed to have done more thorough diagnostics than the first shop. They noted "misfires not present at time of testing" - there was a "P0301 cylinder 1 misfire detected" in the stored codes along with the P0420 cat code. They could not get the O2 sensor off, so ended up replacing the exhaust manifold as well (according to what they said over the phone). The notes however say "replaced upstream oxygen sensor and O2 bung, due to seized sensor in bung" and the itemized parts are the O2 sensor, exhaust manifold gasket, and O2 bushing. So it appears they didn't actually replace the exhaust manifold itself. Knowing little about this, I'm guessing they just removed it in order to get at the O2 sensor. Looking under the car on my garage floor, nothing looks "new" that I can see.

The check engine light is off now. However, they noted that "all passed except catalytic converter monitor, pending code, light is currently off, but likely to return." Now there's a warm fuzzy feeling for you.

They said that the fact that it was so seized up meant the O2 sensor had never been replaced. I'll have to dig back through my records but certainly if it was it would have been at least 10 years ago.

More recently, I had the front right wheel bearings replaced 04/11/22 as noted above. 05/21/21 it was in the first shop for the P3009 HV leak - I read on here or elsewhere that it could be causing by dirty transmission fluid, so I had them "power flush" the transmission. I recall that the big red triangle returned after that, around November, but went away on its own (weird?). So we've gone at least 6 months without the P3009 returning. 12/09/20 a bit further back we had the inverter coolant pump replaced after a P3130 out on the highway. 07/31/20 was when we had the ignition coils (and plugs) replaced. This was a previous time when the check engine light came on. They charged me for a diagnostic but did not note what the code was. The check engine light came on again shortly afterwards and I noted a strong solvent smell under the hood, took it back to the shop, they said it was because the throttle body was dirty, they apparently cleaned that but did not charge me for it, so I don't have any specifics on that one either other than my written note.

I suppose all of this is just normal for a car at the end of life. I'll admit to not adhering to the "maintenance schedule" after about 150k miles, just oil and fluid changes and fixing whatever comes up. The big problem we now face as I see it is that the most recent shop said they can't find an after-market catalytic converter and I'm not sure if even the OEM one is available. So if the P0420 comes back, we have until September before our next emissions test. We're in CO and they require CA stuff. I'd sure like to take old Moonbeam to 300k but we'll see. It still drives and looks great, and I much prefer the sedan design over the post-2004 hatchback. I suppose we'll just go all-electric next anyway... we installed solar panels last year.

 

The check engine light came back on, two days after the O2 sensor was replaced.