Question about 2000 Toyota Corolla

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2001 toyota corolla check engine light is on. Code reads P420 low threshold. Changed front and rear Oxygen sensors reset the light and drove the car 50 miles and the light came back on. the car will not pass an emissions test in wash. state with the light on. Please help! thanks

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  • Anonymous Aug 07, 2008

    I have a 1999 corolla with the exact same problem. Please let me know what you find out.

    Bob

  • Anonymous Sep 10, 2008

    I had the same problem with a 2000 Corolla, replaced oxygen sensors at a price $500, drove a couple of days, light came back on. Dealer could not guarantee any fix. Can I ignore this warning light? Please share your experience. Appreciate!

  • TAISIN818 Mar 30, 2009

    Identical situation as jerimoshier but in california.

  • TAISIN818 Apr 07, 2009

    Ive been staring at a dull orange check engine light for about 2 years now. The car however, drives, smells and sounds just fine. As registration was about to expire, I took the car for a smog test in California where the smog technician informed me of error code reading P0420 (something about a threshold). I learned by way of fixya that this error code refers to the rear oxygen sensor. So, I disconnected the battery, removed the factory rear oxygen sensor, bought a universal replacement rear oxygen sensor and installed the replacement. Two days later, I took the car in to be retested at the same station; and the car passed! A couple weeks have gone by now and my car is proudly wearing fresh registration tags. Suddenly the check engine light has come back on today! Now what?

  • Anonymous Apr 08, 2009

    I have the same code on my Corolla. As far as I know, it's not damaging to the car, just an emissions issue. My understanding is that it could be O2 sensors or the catylitic converter. Maybe a combination of the 3 (there are 2 o2 sensors). I replaced one and it wasn't the problem. I'm going for the cat replacement as this is probably the problem.



    Good luck everyone!

  • Anonymous Mar 14, 2014

    i hav a 1997 toyota corolla code P0125. I've changed the thermostat, and the coolant sensor and reset it. and the engine light still comes on and it reads P0125 code. Please help!!

  • Anonymous Mar 14, 2014

    I have a 1997 Toyota Corolla code P0125. I've changed the thermostat, and the coolant sensor and reset it. and the engine light still comes on and it reads P0125 code. Please help!!

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Mass Air Flow sensor! Just remove your air filter cover. Go to Auto Zone and purchase some solvent. Spray past the air filter and it should correct the problem. If not, replace the Mass air-flow sensor. This is commonly mistaken for an oxygen sensor. Mine did this and I followed these steps and it fixed the light. BTW, the light stayed on for 5 years before I fixed it. Never hurt anything. It's normally something to do with emissions. Dummy lights. :)

Posted on Mar 30, 2009

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Unfortunately I had the same exact problem on my 2001 Toyota Corolla. I also replaced both oxygen sensors with no success. I actually had to replace the catalytic converter. Worse I had to go thru the dealer . Midas told me they won't replace Corolla converters . At least the problem is solved.

Posted on Nov 16, 2008

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With the MAF use CRC electronic cleaner. it is designed for this use and will not leave residue. it should work but either way this problem will not be bad. i have run years and 1000s of miles with that engine light on due to a bad o2 sensor. Luckly i dont live in an emissions state.

Posted on Aug 09, 2009

I think it can be you gasket on the sensor just below the manifold

Posted on Nov 02, 2008

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How to fix the problem related to engine light on code PO 420 of Toyota corolla 2000, Engine 1.8L - golden03


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I have an engine light on to my Toyota Corolla 2003; diagnosed by the autozone OBD11 SCANNER P420. I replaced the oxygen sensor and the light deleted. After driving the car for 30 miles the light came on...


Hi

The trouble areas associated with this code are as follows:
1)Gas leakage on exhaust system
2)Oxygen sensor
3)Heated oxygen sensor
4)Three–way catalytic converter

The idea is to check for exhaust leaks first then the front sensor and rear sensor. If these all check out then the cat needs to be replaced.

Hope this helps. Thanks for using fixYa

Nov 05, 2009 | 2003 Toyota Corolla

1 Answer

PO420


Not just a faulty catalytic converter will trigger this code. Many other issues can trigger this code.

  • Leaded fuel was used where unleaded was called for
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
  • Delayed spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible solutions:
  • Check for exhaust leaks at the manifold, pipes, catalytic converter.
  • Replace Coolant Temperature Sensor
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2)
  • Replace the catalytic converter
  • Have timing adjusted


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The code PO420 came on for my 2000 Toyota Camray ......


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1 Answer

Engine code p0421


Catalyst System Efficiency Below Threshold (Bank 1)

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold
/ catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings


To Do list

* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
* Inspect the downstream heated oxygen sensor (HO2), replace if necessary
* Replace the catalytic converter

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