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02 monty carlo 3800 nsc , check eng lt code p0420, replace cat converter, should i change the 02s

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P0420 - 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.

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: 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.

02 monty carlo 3800 nsc - 2be1e85.jpg

Fig. Removing and installing HO2S1-3.8L Engine

Finally, YES, I suggest replace the O2 sensor; the heated oxygen sensor may be difficult to remove when engine temperature is below 120°F (48°C). Excessive force may damage threads in exhaust manifold or exhaust pipe.

Good luck.

Posted on Oct 18, 2010

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Those cars are notorious for engine problems unfortunately. The exhaust valves tend to warp from heat after around 200k miles. Therefore throwing the O2 codes. Best outcome and cost effective thing I can recommend is replacing the engine with a new or low mileage engine.

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this is what i found for you

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 sensorwaveform reading should fluctuate. The downstream O2 sensoricon1.png 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.

Most likely, the aftermarket cat they installed is a high flow cat. This commonly causes issues with throwing that code on our cars. You can either have an OE cat installed, or just have the code deleted from the PCM by someone that has a tuner.
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code p 0420 refers to cat converter system bank 1---efficiency below threshold====causes ---cat converter---wiring---HO2S (heated oxygen sensor, the one in the exhaust manifold before the cat converter)
code p 0300 is random misfires
I would suggest changing the HO2S as it reports to the ECM on exhaust content so that the ECM can adjust the air/fuel ratio
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Hello and welcome. the code P0420 is for a bad cat. converter. replacing 02 sensors wont repair this code. the code simply's means that thje cat. converter is not working as designed and will need to be replaced.

The PCM determines the oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

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I have a 2000 tacoma 3.4l with double cats bought from california. I am throwing a P0420 code. Would that mean that my cats would be bad or the O2 sensor?


the code itself is for catalyst efficiency. however, the cats are monitored by the 02s. there should be some basic diagnostics performed on the 02 sensors before condemning the cats. monitoring with a scan tool, your front (bank 1) sensors should rapidly switch from .2V - .8V . this indicates that the engine is operating correctly. if the cats are functioning correctly, the rear (bank 2) sensor readings should remain fairly steady, aprox. .4V . this indicates the cat is working. if the rear 02's are a mirror image (or close to) of the front 02 readings, this indicates the cats are not performing their job, the 02's are functioning correctly, and replacing the cats will fix your problem. good luck!

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Ford Explorer 1998 4.0 sohc Diagnostic trouble code P0420.. I was told two different definitions. 1- oxygen sensor and 2 left side catalytic converter


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Change the 02 sensor down by the cat first,( it's cheaper than a cat), see if that clears the code, if not, then go for the cat. You will already have a new 02 sensor, so that will eliminate those two. Hope this helped.

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