I have a '97 Lexus ES300 (105K miles) and got a check engine light 2 days ago. I got it checked and it comes up with a P0420 code. The code description says "Catalyst System Efficiency Below Threshold (Bank 1)".

As I understand, it could be due to a defective catalytic converter, damaged O2 ensors, contamination of converter due to incorrect sealant, fuel, or fuel additive.

I had 2 repairs/services done on my car in the past 6 weeks:

1) I had replaced the flex pipe and the catalytic converter about 6 weeks ago. The flex pipe was leaking and the exhaust was making a lot of noise. My mechanic advised that I also replace the cat. I got the aftermarket (generic) flex pipe and cat.

2) I had a service done a week ago. I got the transmission liquid flushed, radiator coolant flushed and fuel injection service done.

My guess is I am getting this code due to something gone wrong in either of the above 2 repairs/services. The most obvious cause would seem to be the first one, but I don't know if I can rule out the second one as that was the most recent one ( a week ago).

I would appreciate if someone could help me with this. Could the second service cause P0420 to be set? Or is it possible that I just got a bad converter in repair 1? If yes, then would it take upto 6 weeks for the code to be set?

Any additional advice or suggestions would be appreciated.

Thanks,
-MB

 

The problem stems from you choosing the generic flex pipe w/ cat. It is not up to the same strict standards that Toyota/Lexus needs. The O2 sensor is saying the cat isn't performing correctly. Trust me.....I had the same problem as I too went with the generic pipe ($700 v. $1200 for OEM). I went through all the regular diagnostics - checked the O2 sensor including the resistance in Ohm's. To correct the problem after fooling with it for a couple of months was to solder an O2 sim inline with the sensor - BINGO! No more check engine light and car runs perfectly.

 

most likely maf sensor

 

MAF sensor would throw a different code

 

ivory98es3 has got it.

We see a lot of P0420 on vehicles here, bad high sulfur gas, in turn that affects the catalytic converter, and this doesn't go unnoticed by the O2 sensor after the cat. As ivoer98es3 mentinons, one checks the O2 sensor's resistance (in my RX300 which is a similar engine, the range is 11 to 16 ohms).

Now despite all of my unorthodox piping, I have yet to suffer it, but since the O2 sensor functions via a wavelength, the piping can affect it. Usually, if we see P0420 detected several times after the O2 sensors checks out, we go ahead and replace the cat. Generally, we find such cats have sulphur deposits.

 

I had this same problem about six months ago on my 99. I was told that there is actually three separate catalytic converters. I had one go and then a month later the other one went. Dealer wanted over $800 i believe Mienikee did it for $300

 

Thanks everyone for your insights, suggestions and advice.

I just wanted to post an update that my check engine light went away
this evening. I did not do anything yet. I went to work in the morning
and the light was on. When returning from work, I turned on the car
and it was gone. I have made a couple of short trips since then
and the light has not come back on. Based on this occurence, I would
again have to lean on the experts here to understand what this means,
and if it may have temporarily disappeared and could return again.


Another problem that I didn't bring up earlier, as I did not want to
divert attention from the CEL problem, was that my fuel mileage had
dropped since I had replaced the catalytic converter. Previously I
would get about 24mpg and after replacing the converter I am getting
around 20mpg. I noticed the drop in mileage after replacing the cat
and before the fuel injection service. I have to ask this dumb
question, but can the replacement of the cat. converter cause a drop
in the mileage too (i.e. if I have a bad cat.) ?


Thanks everyone for your help.

 

My check engine light came back on. So in a matter of the past 2
weeks, the light came on, then went off and came back on.

I paid $250 for the flex pipe + converter, and yes, this is most likely a universal converter and not a direct fit as the mechanic had to weld it in.

I did take my car back to the mechanic (the light was off that time),
he inspected and did not find any leaks. He still welded it again just
to close any minor leaks that may be present. The light came back on a
couple of days later, so don't think the second round of welding did
any good. Other than this, he does not see anything wrong in 'his'
converter.

What are my options at this point? Do I have to now go and buy a new
OEM converter?

 

check out all the 02 sensors waveform

 

You really should get a OE converter. Id take it to Lexus and ask them to reflash the PCM.