Question about 1999 Lexus ES 300

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I have a Lexus ES300 with a code P0420 showing a bad catalytic converter or Fuel System Malfunction. It appears the converter was replaced with a muffler but the light remained on. When I'm at a stop, the car smells like its running rich. Do I just need to get someone to reset the code or did the replacement cause another problem?

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Usually the ecu never gives a code on a catalytic converter, but rather the o2 sensors. it seems much more logical that it is the o2 sensors than the cat. fixing the problem usually makes the light go away but not always, so resetting the computer is adviced. also, resetting the ecu would not change the way the car runs because it should be working normally, which means, if its running rich, the problem still exists. like i said, most likely it is the o2 sensors, especially because they go out fairly commonly. hope this helps, good luck.
p.s., if the muffler did get replaced, need high flow o2 sensors, but that depends on the muffler. also, i posted alink to a lexus forum so when ever you have problems with your car, you can ask there:
http://www.clublexus.com/forums/performance/35057-ecu-reset-remove-fuse-or-disconnect-battery.html

Posted on May 19, 2011

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  • jerrysd1 May 19, 2011

    Actually, I replaced both O2 sensors a few weeks ago which got rid of another code that came on. Thanks for the tip, will look at the link!

  • Jorge Majano May 20, 2011

    you might want to try what the guy beneath me said, it makes sense.

  • Jorge Majano May 20, 2011

    i found a link this link on your cat. its not on your muffler, its before. it actually looks to be right under the motor. hope this helps
    http://www.clublexus.com/forums/es300-an...

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Hi, a car that's running rich can also cause this code to set. You might want to check your fuel pressure regulator for a torn diaphragm. The regulator is on top of the engine connected to the fuel rail and looks like the picture below. Pull the vacuum hose off the regulator and plug the hose with a golf tee. Then start the car and watch for any fuel that spits out of the vacuum port of the regulator. This indicates a torn diaphragm. Please let me know if you have questions, and thanks for using Fixya.

I have a Lexus ES300 with a code P0420 showing a - jturcotte_503.jpg

Posted on May 19, 2011

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I have a llexus wirth this problem also and found the air cleaner hoses were broken also fuel pressure regulator screw was all the way out I replace all this parts and reset the check engine light test drove the vehicle and dint found any other problems sofar good luck hope this will help since every vehicle is different test all related parts and functions is advise

Posted on Jul 04, 2013

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P0420 is Catalyst System Below Efficiency (i.e. catalytic converter on the exhaust system.)
To check your catalytic converter, get a very light, small finishing nail hammer and tap the catalytic converter lightly, if you hear rattling inside it then it's shot. If your exhaust pipes are rusted through anywhere, get it all done at the same time. You can DIY using parts from Walker Muffler... schematics are available on line, get new hangers too!

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P0420 OBD-II Trouble Code Technical Description Catalyst System Efficiency Below Threshold (Bank 1) What does that mean? This diagnostic trouble code (DTC) is a generic powertrain code. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model. So this engine code article applies to Nissan, Toyota, Ford, GM, etc. The catalytic converter is a part of the exhaust system that looks kind of like a muffler, although it's job is very different from that of a muffler. A catalytic converter's job is to reduce emmissions from the exhaust. 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). Catalytic converters are not normally a "wear" type item, that is they are not deisgned to wear out and need replacement. If they have failed, it is likely due to something else that caused it to fail. Symptoms The main symptom to the driver is the illumination of the MIL (malfunction indicator lamp). You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle. Causes A code P0420 may mean that one or more of the following has happened: Leaded fuel was used where unleaded was called for A damaged or failed oxygen sensor (HO2S) Downstream oxygen sensor (HO2S) wiring damaged or connected improperly The engine coolant temperature sensor is not working properly Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe Retarded spark timing The oxygen sensors in front and behind the converter are reporting too similar of readings Leaking fuel injector or high fuel pressure Cylinder misfire Oil contamination Possible Solutions Some suggested steps for troubleshooting a P0420 error code include: 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 (HO2S), replace if necessary Replace the catalytic converter Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily. We strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in our forums where folks replaced the cat with a cheaper aftermarket one only to have the code return shortly thereafter. One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

Read more at: http://www.obd-codes.com/p0420
Copyright © OBD-Codes.com

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The P0420 code means that the catalytic converter efficiency is below threshold. Since there are two oxygen sensors, above and after the catalytic converter, they together generate signals which are used to monitor the efficiency. However, if the catalytic converter is bad, the sensor after the converter will generate steady results, without any fluctuations.
You can replace the catalytic converter to overcome this problem. However, there are chances of the P0420 code to come up if there is any leak in the exhaust or the downstream o2 sensor (after the converter) is bad. Ideally, before spending on cat, you should try replacing the downstream sensor, which you can buy at low price from online stores like oxygensensorworld.com and also inspect for any small leaks in the exhaust system. Since the converter can cost you more, think of replacing it only if needed.

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This Code means your Catalytic converter is not working like it should. You may have a problem on Bank 1 or the O2 sensor is giving you false readings. At a minimum you should either replace or have the O2 sensor tested to verify that the Catalytic converter is working properly.

If the O2 sensor is accurately performing its job, then have the engine scoped before replacing the Catalytic converter.

You should have received a defect Code for other components which could foul the Catalytic converter. This is why I recommend getting the engine scoped. If other defects were corrected before you purchased this vehicle, then the Catalytic converter may be able to recover by itself.

The Converter costs so much that you should verify things like low compression on the number 1 bank.

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The PCM determines this by the readings it gets from the oxygen sensors before and after the catalytic converter. The code can be caused by a bad catalytic convertor or a bad heated oxygen sensor. It can also be caused by the fuel system if a malfunction there is trying to make the fuel/air ratio too extreme (lean or rich) to make the oxygen sensor attempt to work outside of their specified range.

I would recommend taking your vehicle to your local trusty auto repair facility and having them diagnose the issue and recommend a course of action. Don't be afraid to take it to multiple places to get second opinions.

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What does that mean? 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.


Symptoms You will likely not notice any drivability problems, although there may be symptoms.
Causes 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
Possible Solutions Need a catalytic converter? Visit our catalytic converter shopping area for the lowest price on your replacement converter!
Some suggested steps for troubleshooting a P0420 error code include:
  • 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
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

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With that code you most likely have to replace the catalytic converter.You can try to reset the light but it will come back in time.

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