2002 Prius w/ 287k miles, P0420

Hi all,

I've been lurking here for a few weeks now trying to find a solution for my P0420 code because I need to get a smog check fairly soon.

So a little backstory:

Around mid-April of this year, the 12v battery died due to the car not being used frequently (worked from home). I changed the battery ~2 years ago with a Mazda Miata battery that I got from elearnaid. This time, I decided to go with the more expensive D51R Optima yellow top battery and we were back in business... for ~100 miles.

After driving the car daily to and from work (~12 miles each way) for a few days, I decided to fill up at Costco and got a CEL after 10 miles. Now, I've gotten a CEL a few times -- all P0420 -- in the past 3 years since I got this car (got it from a buddy).

• The first time was in 2018 - the mechanic reset the code and told me there was a crack in converter so he suggested I go to a muffler shop.
• I go to the muffler shop two years later in 2019 because the CEL didn't come back on, again after filling up (may have topped off). I passed the emissions test in 2018. So I go to the muffler shop and they welded a patch for the crack which made a difference in how my car sounded. Apparently, it sounded like a diesel truck when I took it in
• Fast-forward a few months and I get another CEL, again after filling-up. I drove the car for 200 miles and the light turned off.

This brings us to today. I drove the car 300 miles after filling up, but this time the light didn't turn off. I reset the light by disconnecting the 12v battery and the light came on in ~100 miles. I take the car to the mechanic and he pointed to the catalytic converter (for obvious reasons). I suggested we switch out the upstream first sensor after reading some threads here and he was on-board with that after checking out the sensor data (last Friday 07/03). I drove the car 100 miles, filled up at Costco (idk why I keep doing this), reset the trip meter and in less than 10 miles the CEL came on this past Thursday.

I attached some O2 sensor readings that I got on Friday but I'm not quite sure on how to make sense of it all. If I understand correctly, sensor 1 (upstream) should fluctuate which it does and sensor 2 (downstream) should be flat. In the "O2 sensors" picture, I have the car parked and have the air on for a few minutes and then I turned the air off. I disconnected the 12v battery again and will be taking it for a drive today to see if the light turns back on.

Thoughts on what I should do before getting a new catalytic converter installed? For what it's worth, I live in the Bay Area and converters/installations go for ~$2000 from what I've heard. The car has no oil consumption issues. I'm hesitant to spend the value of the car for a part that's highly sought after here (lots of stolen converters!)

Anyway, thanks in advance if you've read this far. Hoping this thread helps others with their problems.

 

Hi Josey,

Good points all around, and I appreciate the feedback.

Apparently, the upstream readings before the change were also funky therefore we decided to change that first. I should mention that he replaced the upstream sensor with a Denso O2 sensor. I made sure to ask about that before the change because I read about cheap aftermarkets so I wanted to rule that out. Now, I didn't actually get to see what technician put in but I trust this shop as I've been going here for years. I'm not sure if it was way up in temperature but I drove to Oreilly's (5 miles, 10 minutes), parked, then I started the car and performed testing for the next 10 minutes. I attached some more pictures with temperature readings, hopefully that addresses your concern.

Since it keeps happening after I fill up, I feel like it may have something to do with the EVAP system or maybe it's just a coincidence. I'm certainly glad that the catalytic converter seems to be doing it's job. The car honestly runs fine and I haven't noticed anything out of the ordinary like loss of power or drop in fuel economy. I'll ask my mechanic what he thinks could be throwing off the sensor.

 

Update:

So I drove the car until the trip meter hit 150 miles (my CEL usually comes on around 100 miles). Last Tuesday, I took it to the smog shop for STAR certification but the catalyst monitor wasn't ready so I found the drive cycle (drive pattern 3 in attached file) and did that the next day. On my 2nd attempt of the drive cycle, my CEL came on (~170 miles).

Feeling hopeless, I go to the muffler shop to get the cat converter checked out and it turns out there are more cracks and it'll be $1500 to replace which is reasonable. I wasn't actually there to see what cracks they were talking about, but this time there is no use in welding a patch. They reset the code so now it's pending. Since I have nothing to lose, I purchased Cataclean for $25 and have been driving the car with the hopes of getting the catalyst monitor ready anyway.

Yesterday I went to Oreilly's to see if the cat monitor was ready (it was not) after driving on freeway speeds for 20 miles. While I was there, I took some more O2 sensor readings (see attached files) that I was hoping someone can help me interpret. The downstream sensor (red) in the idle picture is not scaled properly and actually hovers .8v The scale changes when I rev the engine up. Now that I think about it, I should have included the RPM graph in this picture. Anyway, is the downstream sensor responding properly when I rev the engine? I'm debating changing it anyways as the last attempt before getting a new cat installed or maybe parting ways with the car

 

Last update:

Changed out the cat converter and downstream censor. This is what a good cat converter and O2 sensors look like.