Question about 1999 Chevrolet Silverado 1500

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Relaced all O2 sensor at same time good OEMreplacements. Replace catalytic converter 3months ago. Still engine lisght read check engine. Yes I turn off the light with obd 11 meter and still comes back. WENT OVER ALL VAC LINE AND REPLACE.

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  • Thomas Woop Sep 06, 2010

    TELL ME SOMETHING I DON'T KNOW. DID YOU READ MY E-MAIL. I USED A SCANNER

  • Thomas Woop Sep 07, 2010

    READ MY ORIGINAL QUESTION!

  • Thomas Woop Sep 07, 2010

    IT WAS CHECK WITH A SCANNER AS MENTION IN ORIGINAL E-MAIL. CODE P0420

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4 Answers

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  • Master
  • 439 Answers

What is the code the computer is shooting out at you?

Posted on Sep 08, 2010

  • Matthew Boulden Sep 09, 2010

    Common Problems That Trigger the P0420 Code


    • Inefficient Catalytic Converter(s)

    • Defective Front or Rear Oxygen Sensor(s)

    • Misfiring engines

    Common Misdiagnoses


    • Oxygen Sensors
    You need to replace the spark plugs, check or replace the wires. there will not be any adverse effects on the motor or vehicle. It means the vehicle is putting out too much emission due to the catalytic converter not working as it should or the misfire.

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  • Chevrolet Master
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USE A SCANNER TO SEE WHAT CAUSING CHECK ENGINE LIGHT TO COME ON.YOU COULD HAVE CYLINDERS MISFIRE OR EVAP LEAK THAT WILL CAUSE CHECK ENGINE LIGHT TO COME ON.

Posted on Sep 07, 2010

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What is the code or codes that keep coming up? P0171 P0173? If suspecting a vacuum leak causing a lean condition get a can of throttle body cleaner or induction cleaner and spray around the intake and listen for rpm changes.Any additonal information would be helpful.

Posted on Sep 07, 2010

  • Jonathan S
    Jonathan S Sep 07, 2010

    Are the intake gaskets ok?Was referring to the intake itself leaking vacuum. I understand you have replaced the vacuum lines from your previous post is the brake booster holding it's vacuum internally.Just trying to get to the source of your issue and what are the codes that keep coming up.

  • Jonathan S
    Jonathan S Sep 10, 2010

    I know the o2 and cat were replaced from your posts.Were you able to verify that the O2 were switching especially the post converter O2? Is there a possibilty that the new cat may be defective?

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Hi, you don't have to carry out any repairs on you car when the check engine light comes up without scanning it first.

Check engine light requires you to have the engine CPU scanned for trouble code's. This can only be administered with the assistance of a diagnostic computer scan tool called OBD 11 scanner. You can purchase this tool, at your local auto parts store, or you can just have the scan ran at your local auto zone parts store, free of charge. Once the diagnostic scan is administered, you will be able to isolate the issues that have caused the check engine light to illuminate.

This scan tool will display a print out code and the location of the issue, on the engine, as well. There are Many issues that will cause the engine light to illuminate, but the most common cause will be a failed monitoring sensor, or sensor harness failure (separated power wire, or ground e.t.c).....

Posted on Sep 06, 2010

  • Tunde Autos
    Tunde Autos Sep 07, 2010

    If you used the scanner, what code did the scanner produce...

  • Tunde Autos
    Tunde Autos Sep 07, 2010

    ......

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1 Answer

Code p0420 acura m'dx


1996-newer (OBD-II) vehicles use oxygen sensors and a catalytic converter to reduce emissions. There is one O2 (oxygen) sensor in front of the catalytic converter (called upstream), and one behind it (downstream). When the engine is warmed up and running normally the upstream oxygen sensor reading fluctuates and the downstream one is fairly steady. In the case where both readings are fairly steady, a P0420 check engine light code is set.
Possible Causes: Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter
Diagnostic Help:
To diagnose a P0420 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilliscope to check the waveforms of both upstream and downstream O2 sensors. The front reading should vary, the rear should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component concerns. See our article "Automotive Circuit Testing 101", if you need more assistance with this. Remember to refer to an model-specific repair manual for proper instruction.

Jan 20, 2014 | 2005 Acura MDX

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I have Service engine light on for my nissan 2001 maxima. Dealer ran the diagnostics and said Bad o2 Sensor. Today , the Service engine light is gone.. Do I need to replace the part anyway or wait for the...


if it comes back, yes you will need. without pre o2 sensor your car wont idle, it will use lots of fuel and you will face drivability problems if it is post o2 sensor, it is there to check if catalytic converter is ok or not(it won caue drivability problems but may consume little more fuel to run rich and to save catalytic converter). if sensor is broken you need to replace it. if you have error for post o2 sensor but it is ok, this means your catalytic converter is going bad.

Nov 18, 2011 | Cars & Trucks

2 Answers

What is engine code P0420


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.

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


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.

Hope this helps (remember to rate and comment this answer).

Jun 28, 2011 | 2005 Chevrolet Cobalt

1 Answer

Code P0420 is it an oxygen sensor or the catalytic converter?


The code po420 is the cat efficencey which means the cat is bad but it could be a bad sensor, you will need to have the o2 sensors checked to see but lots of techs say a good tech should know if an o2 is bad but i will tell you i have had some that test good and end up being bad. they tested good at the time i checked them, a good rule that i follow any car with over 80,000 miles on it with a bad converter gets new o2 sensors, this is to help customer and us from comebacks, a weak or slow o2 will destroy the new converter, it may test good at the time of the new converter but then a few months down the road it gets lazy and after a while some ones not going to be happy having to buy another converter after just doing one a few months ago or even a year ago, seen it ,done it, many times,if i were you i would replace the o2 sensors and reset the light and go from there , if light stays out good if not replace the converter, all you are doing is doing it in two steps and you may get lucky and not have to replace the converter good luck.

Mar 19, 2011 | 2003 Honda Element

1 Answer

Engine code 430 for 2001 toyota highlander


Possible Causes:
Oxygen (O2) sensor not working correctly
Damaged or leaking exhuast (pipe, manifold, catalytic converter)
Damaged catalytic converter

Diagnostic Help:
To diagnose a P0430 trouble code we begin by checking for exhaust leaks under the vehicle. Then use an oscilloscope to check the waveforms of both upstream and downstream O2 sensors. The rear reading should vary, the front should be steady. Replace as necessary. Check the rear Heated O2 Sensor. Check the catalytic converter, replace if necessary. If necessary, check for wiring and component. hopes this helps thanks

May 30, 2010 | 2001 Toyota Highlander

1 Answer

I have a 2005 Chrysler Town & Country with almost 100,000 miles. The check engine light came on about 2 months ago and the code said the O2 sensors needed to be replaced. About a month later, the...


there are no codes that say replace O2 sensors,or remove converter,Thats just somebody's guessing,When it says O2 sensor that tells the mechanic to look for trouble in that area,wiring or faulty readings,And there are four O2 sensors.

Jan 20, 2010 | 2005 Chrysler Town & Country

2 Answers

Trouble codes


You mean P0420? You need a new cat converter. The "Mechanic" couldnt tell you that?

Nov 25, 2009 | 1998 Chevrolet Venture Cargo

1 Answer

I have a P0420 code on a nissan help me fix it


catalytic converter efficiency below threshold bank 1 possible problems are engine coolant temp sensor or o2 sensor not reading properly or front and behind catalytic o2 sensors read similar-- ****** spark timing--damaged or leaking /exhaust manifold/catalytic conv/exhaust pipe. check for leaks or damage to exhaust repair or replace as necessary

Nov 15, 2009 | 2005 Nissan Altima

2 Answers

Changed both o2 sensors and front catalytic converter 2001 v6 tacoma 4wd dbl cab check engine light came back on initial scanner diagnosis catalytic efficiency bank 1 any ideas? could it be back converter


if the converter was bad the engine would loose power . alot of times the check engine light will come on for the emmitions check youre gas cap some vehicles will send a signal to the computer if the co is not right and by changing the gas cap solved the problem .

Nov 14, 2009 | 2001 Toyota Tacoma

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

Jan 04, 2009 | 2001 Hyundai Sonata

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