Hi, I have a 2000 camry 4 cyl. with 153,000 mi. I had a hole in my exhaust pipe and needed to replace the front flex pipe assembly. I had it done at Meineke with after market part. I know, Toyota wanted $1,300 for the part excluding labor and Meineke did for about $400.00 total. About 1 week after replacing, the check engine light came on with code P0420. I cleared the code 2 times and it came back.
I brought back to Meineke (also had it read by 2 other service places) and was told I had to replace the O2 sensor before Cat. (air fuel sensor on Cal. emissions). I replaced myself and cleared the code. I drove about 8 miles with no problem. The next morning on way to work, I got about 10 minutes from home and the check engine light came back, same behavior as before replacing sensor. Could this be the rear sensor needs to be replaced?
Thank you for any help.

 

The computer compares the output of the oxygen sensor located before the CAT with one after to determine if CAT efficiency has deteriorated.

If the CAT is OK, the output of the sensor after the converter switches slower then than the waveform of the sensor before the CAT. If both sensor outputs vary at similar rate CAT performance may have deteriorated or there is a problem with the sensors. Code P0420 is then generated.

The computer has no direct way of testing the CAT; as such the problem could also be the sensors. A shop can do further testing to narrow down the problem.

Problem areas include exhaust system leaks, sensor(s) or CAT.

Keep in the mind the computer has trouble codes for both front and rear sensor troubles. As such if a sensor were the problem another code might pop up another code and the P0420 code.

 

Thank you for the explanation.
I am going to change the rear sensor also then see if there is any change, since the mileage is up there. If not, I think you are correct in getting a pro to troubleshoot.
Thanks again.

 

Jcamry,

Did replacing the rear O2 sensor work?

My 2003 SE has the same P0420 code; from what I was able to research online is that unless you get codes P0136 or P0137 your pre-catalytic converter or after converter O2 sensor need to be replaced. Code P0420 is displayed when certain parameters obtained by both sensors are not in line with the PCM configuration. It appears that the onboard computer constantly reads this information and if its below efficiency the CEL comes on.

I will be ordering a replacement Catalytic Converter from Magnaflow (Universal Converter).

http://www.magnaflow.com/

Follow the link, select products, catalytic converter, universal fit, select make and model, write down the part number and search in amazon.com; it’s $81.80 (for me 2003). Going to take it to a local muffler shop for installation.

I was about to order two universal Denso O2 sensors online until I did more research online.

Visit http://www.densoaftermarket.com/
Click on find my part,
O2 Sensor
Select your make, model, year, etc.
Write down your part number(s) and search in amazon.com. I think the front Denso O2 was $40 bucks and rear O2 was about $60. YOU CAN’T BEAT THAT MY FRIEND!!!

See O2 Info Below:


SHORT TERM FUEL TRIM from O2 Bank 1 Sensor 1 & Sensor 2

Short Term Fuel Trim represents a short-term correction to fuel delivery by the PCM in response to the amount of time the fuel control oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly remained less than 450 mV; indicating a lean air/fuel mixture; short term fuel trim will increase into the positive range above 0% and the PCM will add fuel. If the oxygen sensor voltage stays mainly above the threshold; short term fuel trim will decrease below 0% into the negative range while the PCM reduces fuel delivery to compensate for the indicated rich condition. Under certain conditions such as extended idle and high ambient temperatures; canister purge may cause Short Term Fuel Trim to read in the negative range during normal operation. The Pam’s maximum authority to control fuel trim allows a range between -11% and +20%. Fuel trim values at maximum authority may indicate an excessively rich or lean system.

O2 SENSOR - O2 Bank 1 Sensor 1 & Sensor 2 (mV)

Represents the fuel control exhaust oxygen sensor output voltage. The voltage should fluctuate constantly within a range between 10 mV (lean exhaust); and 1000 mV (rich exhaust); while operating in closed loop.

 

Here is my video on:
How to Replace Oxygen Sensor Toyota Camry