Question about 1998 Volkswagen Jetta

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Po420 haunting me 03 jetta 1.8t

Replaced both o2 sensors and cat. need to pass emissions in az light has to be off dealer says drive car for 200 miles to cycle through computer but light comes on at 40-80 miles what do i do

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  • brian051307 Feb 04, 2010

    codes p0420

  • brian051307 Feb 04, 2010

    new engine and plug plus coils now what

  • brian051307 Feb 04, 2010

    no car runs great and idles great no problems

  • tim mcgrath May 11, 2010

    what codes get? takes mostly 30 miles to check system

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Are you using stock catalytic convertors? Any other upgrades?

Posted on Feb 04, 2010

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Po420 is a generic obdii code which is cat converter sys - efficiency below threshold bank 1, which on vw's, should get oem code to fix right, does engine run rough?

Posted on Feb 04, 2010

  • tim mcgrath Feb 04, 2010

    check the pcv valve hose very known for broken hose can cause running lean cause of vac leak,

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You're going to have to have the codes read and find out what is setting that light on. After that problem is repaired, the car must indeed complete a drive cycle, not nessacarily 200 miles. This lets the computer run all it's tests and set all its readiness monitors. If you know what the code number is now, post it and I'll tell you the most likely cause, and repair.

Posted on Feb 04, 2010

  • Dave  C
    Dave C Feb 04, 2010

    Sorry, just saw the code in your title, you need spark plugs. The new cat can't handle the large amount of emisssions produced by a missing engine, possibly a bad coil. How many miles on the engine? If it is worn badly...same problem.

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The parameters VW has defined for the setting of code PO420 often can not be satisfied by the installation of an aftermarket converter. Aftermarket converters are appreciably less expensive for a good reason, they are not constructed of the same quality as the original, i.e. they do not contain the high levels of precious metals found in the originals. If you don't believe this check the scrap value of an aftermarket converter compared to an OE, big difference. Hope you can use this info to solve your problem.

Posted on Mar 23, 2010

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16804 P0420 Catalyst System, Bank 1 Efficiency Below Threshold
I'll see what it means when I get back a bit later.

Posted on Mar 04, 2010

  • Ed Paje Mar 05, 2010

    It might seem out of place but a check on the Coolant Temp Sensor might be culprit. A haunting is caused by either a bad 02 sensor or an over working 02 sensor. If the ECM couldn't read the correct engine temp it would take this data from the 02 sensor. The data it seeks is either a lean or a rich burn which translates the temp inside the combustion chamber.

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1 Answer

How many miles do i need to drive to have the computer re-ready to accept obd codes?


200 miles is more than enough. p0420 is a cat code meaning the cat in the exhaust is not doing its job. you need to make sure there are no exhaust leaks before the downstream (last one) o2 sensor. if you can get all but 1 monitor to set without the check eng light coming on, you will pass pa emmissions.

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Hello! My long time (recently not so helpful) mechanic told me my engine light was on due to an O2 sensor code. This lasted at least a year and now the car is giving a code for the Cat Converter. Do you...


replace the o2 sensor,, clear the codes ,, and see if that cat code comes back, they work in unison,, so you may just need the O2 sensor, and all codes will clear

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I have changed the exspensive o2 sencor and engine light went on for two months then went out. i think bulb burt out but went to autozone and the code po420 came up. do i just need to change that sencor...


The code means 'Catalyst System Efficiency Below Threshold (Bank 1)' which means the computer thinks your catalytic convertor is not functioning properly. However, since the convertor itself doesn't have any sensors, how the computer knows this is via the O2 sensors (both before and aft of the cat). However, if the O2 sensor is dying or faulty it could give erroneous readings that would trigger this code, and the most common one to fail is the downstream, or Bank 1, sensor. The cat is very expensive to replace and usually that would be the last thing you would want to try. O2 sensor failure is quite common but you said you already changed it so.
Another reason you might see this code is if the internal coolant temp sensor is faulty or not functioning. If the computer thinks the coolant is warmer than it really is, it will run the fans more and keep the engine cooler. A cooler engine produces worse emissions than a hotter one so the cat wouldn't get up to the temperature it needs to be to function properly, thus triggering the code. Again, not an expensive sensor to replace and every dealer should be able to test them with a volt meter before blindly replacing them.

Finally, your original cat (and pre-cat) are warrantied by federal law for 8 yrs/80K miles so if your '02 is under 80K it should have been covered by Acura

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The check engine light came on on my 2004 Pontiac Vibe 1.8. Code PO420 came up on my code reader "catalyst system efficiency below threshold bank 1. Also on the face of the reader the C (cat monitor)...


My first thought would be the converter considering the codes....which will cause the sensors/heaters and other components to fail or read as being bad... I would call a dealer and make sure there are no recalls on the emissions systems and anything to do with the vehicle..
Also it would be worth your while to scan it with a TECH 2 scanner......You may have one I dont know......But I worked for Buick/Pontiac/GMC for 5 years and TECH 2 was a life saver...
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The cat is the primary part for cleaning up your emissions. Like any bad part on a car if it is not replaced in a timely manner it can lead to other parts going bad, such as the oxygen sensors. It can also negatively effect your gas mileage, because the computer determines fuel off of the oxygen sensors. The check engine light should also be on preventing you from passing inspection(if your area has an emissions inspection).

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2 Answers

Code PO420 catalyst system efficiency below threshold


you may have a bad converter, its not cleaning the exhaust, but on the other hand a bad rear o2 sensor can also cause this problem,there is a wat to test but you would need a scan tool or scope,replace the other sensor, if light returns then replace the converter, no money lost, if you were to replace the converter the o2 sensors should also be replaced. you have 165000 miles it would be crazy not to replace sensor and install new cat, so basically your just doing it in 2 steps and may get lucky and not need a cat. good luck.

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PO420 Catalyst System Efficiency below threshold.


Basically the mechanic will make sure don't need a tune up. or have vacuum issues or leakes in the exhaust before the cat. Then he should check the front 02 sensor to make sure its not getting lazy. If all this passes then you will need a new cat. If the O2 sensor has more than 70,000 miles on it it needs to be replaced anyways so you may just want to do this first and hope the light stays off.. The problem with a lean code is you will wear out the cat real fast if you didnt deal with this and kept driving the car. The lean code could have been vacuum leak or a bad MAF sensor or weak fuel pump.

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1 Answer

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


Catalyst Effeciancy below threshold (Bank 1) is the code meaning. Check or replace the oxygen sensors. Some cars have 2 .One is on the front of the cat. the other is the on the back. Check the back sensor . Other possible causes are leaking or damaged exhaust manifold, bad coolant temp sensor.If toyota did not replace both . replace the one on the back of the Caty.

Feb 02, 2009 | 2003 Toyota Corolla

2 Answers

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- I got some details there. And see my comment for Dennis above here too

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