Quote:
Originally Posted by zran22
Last week I bought my gf a 2001 corolla with 120k miles. It threw two codes while test driving it. it seemed to drive fine with no stutter or hesitation. I thought those codes can be taken care of easily but I guess not.... I replaced the upstream (pre-cat) o2 sensor with a Bosch one from autozone, and cleaned the maf sensor already. I used the right cleaner and it did seem that the maf was dirty as hell. After cleaning the maf i took off the neg battery cable only. I also did this after replacing the front o2 sensor. (On a side note I also replaced valve cover gasket and oil pan gasket.) I checked one spark plug and figured the rest were the same. Clean and looking new. I also noticed that they have a thin electrode. After replacing the o2 sensor and cleaning the maf sensor the car was now stuttering and hesitating mostly after a stop sign or red light from a stop. It is very bad trying to take off, as if you have to pump the pedal to go. She had it checked at autozone and the same 2 codes were still on. This was after the replaced parts were installed. Is there another way of resetting the ecu? What do i need to check? Can anyone help me or point me in a guided direction? I am currently living away from her and might have to make a trip to fix it. I want to have my stuff together obviously before I make a trip.
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The PO420 is a common problem on Corollas, your rear O2 sensor is not staying where it should be after the Catalytic converter has gotten to light off temperature... Sometimes it means the cat is bad, sometimes not.... This code was developed to let the driver know that the Catalytic converter was not operating within defined parameters....
The easiest way to check a cat is to run the car to operating temperature and then set the idle at 2500 rpms (I use a matchbook to do this)....
Get an inexpensive laser thermometer (Harbor freight sells a cheap one) and check the temperature of the cat.. Front and back... The exit of the Cat should be 100 degrees Fahrenheit or more hotter than the inlet... If the front is hotter than the rear, then you definitely have a bad cat, that is what the PO420 is being shown for.
You can also test see if you have exhaust blockage (possibly the cat is clogged) by using a vacuum gauge, but that is not what is throwing this code... but is a good backup test...
If your cat is still good, then you can clear the code and some people install an extension plug where the rear O2 sensor screws in... Stops the code from being thrown... This should only be done when you are sure the cat is still good and your emissions are legal.... After you do this, you clear the codes and the PO420 wont come back....
PO420 will not affect drivability, but will cause you to fail an OBDII based emissions testing if you are in a state that requires testing and does not do tailpipe or alternative tailpipe testing.
The PO172 means the front O2 sensor is detecting the fuel mixture being rich...
Check out this thread for more information...
P0420 & P0172 codes
A few things... I do not recommend replacing O2 sensors with Bosch... This is one of those cars where Denso's are really the only thing you should use (yes, they are more expensive)
Check the MAF sensor by disconnecting it from the wire harness and start the engine, if the engine runs better you have a bad MAF.... Sometimes cleaning them helps, sometimes it doesn't.
To reset the ECU, leave the battery disconnected for 10-15 minutes, I think it takes less, but more time won't hurt....
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