2001 Toyota Camry - check engine light on - code p0420 - took to mechanic and he just hooked up code scanner and gave p0420 code - without checking anything else he said rear O2 sensor needs to be replaced? I am due for inspection next month and want to make sure this is fixed properly. Is it always the case that with p0420 code, its always rear O2 sensor? Or can it be something else like CAT, Front O2 sensor, coolant Temp sensor etc. I dont mind changing the rear O2 sensor but I want to make sure that rear O2 sensor is actually the problem. Please help.

Mine is a 2001 Sienna LE. I got the P0420 code and the dealer want to replace an engine control computer for $1600. I've found posts that say you can replace your oxygen sensors, which still costs around $100 each. The P0420 seems to be emissions related. When my check engine light comes on, I run the highest grade, no ethanol gas, and usually within a couple of tanks, the light goes off again. The last time it happened about 2 wks ago, I put in the 93 octane no ethanol premium and a can of Seafoam and within a half a tank, the light went out. Your problem might be more serious, and I don't know what the 1150 code is, but so far that's worked for me.

P0401 indicates that the Exhaust Gas Recirculation Flow Insufficient Detected. This can be because EGR is faulty . As you have replaced this then the problem must be with the vacuum line . EGR valve vacuum line which can be broken or blocked. The vacumn ine must bbe checked for leaks or any blocks using an vacumn pump connected the line.

P0420 indicates that the down stream oxygen sensor reading is not steady and it is connected with the code P0401.

These are egr codes.Replace your egr valve it is most likely stuck closed causing the car to stall.

P0420=Catalyst System Efficiency Below Threshold Bank 1,
The air filter is not the cause here,this is just a code saying your converter is not doing its best,

there is a possibility of a short to the bulb... he is putting the scanner on it and clearing the code,,,,or he is just undoing the battery cable, making light go off for a bit, and then coming back on. Consider taking it to a different mechanic. but if ur check engine light is on, there is a reason its on, or a short to bulb

Can be many things, need to hook it up to scanner.. if u live close to AutoZone or Advance Auto they will check for code free.

P0420 is catalyst system efficiency below threshold (bank 1)

P0440 is evaporative emission control system malfunction

P0441 is evaporative emission control system incorrect purge flow

P0420 indicates you have a catalyst efficiency issue; replacement of the Air-Fuel Sensor should take care of that;
The P0441, P0446 and (I assume P0440); you said 446 twice, indicates a faulty or loose fuel cap; it is not sealing properly.
Please feel free to respond to me if you have any further questions.
A FixYa rating from you regarding my assistance would be greatly appreciated.

P0420 is a cat code. Sometimes, you can replace the upstream & downstream air/fuel & O2 sensors, run some good fuel injection cleaner thru it, and that might fix the problem. If not, you may need to replace the cat. Note that the cat is covered under warranty for 8 yrs/80,000 miles. Toyota will replace it if it has gone bad, for free.

normally you need to change the converter if you have smog test you have no choise if not disconnect the battery for a minute and try it out hopefully it will not come back hope its a fixya thanks for the post pierre