Catalytic convertor below thresold

Hi everyone,
My 2014 VW passat 3.6L has 133KMs.

I have been driving this car with regular 87 gas from last 3 years (premium gas is recommended for the car thou) dumb of me🤦‍♂️

I have got engine light many times but goes off automatically within couple days but this time its there from last one week and its concerning me. Got alarm code P0420 catalytic convertor below thresold. What do you think will be easy fix?
Changing O2 sensors? Since i was putting regular gas from 3 years

I just added Fuel injector cleaner in the tank, went on a drive and engine light is gone FOR NOW.
Do you recommend any additive for catalytic convertor cleaner?

I also had alarm, higher RPM than normal

To find out if the cat is bad, a scanner is used to monitor up-stream/down-stream O2 sensors.



+1 to @Tester , that is the reliable method to check the converter. From what I have seen, VW converters seem to go out at about 100,000 miles, ~160,000 km, yours may be failing early.

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Would you recommend changing oxygen sensors? Never changed before, driving to 133K kms now
How about some catalytic convertor cleaner?

Find out if the cat’s bad first.


Mechanic told me cat will be the last thing he will look it since it is expensive🤦‍♂️

I’ve been repairing cars for over 50 years, and a P0420 code has always been a bad cat.

So if you want to waste your money, go ahead.



The very first thing you do is rule out any leaks in the exhaust system

If there are none, it’s very likely the cat has degraded to the point where the P0420 code is generated

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Your mechanic should test the entire system properly and come up with a determination, not just check things one by one. He should first verify that the engine is in proper running condition and that there are no misfires and that the fuel injection system is running within spec. He should check all the oxygen sensors or air/fuel ratio sensors to make sure they are reporting properly. He should check the exhaust system for leaks, and then he should check the catalytic converter operation to test its efficiency. Except for maybe inspecting the exhaust, all these things can be done from the driver’s seat without opening the hood.

On many cars, the cat can be unbolted, which would allow you to try cleaning it with carburetor cleaner or a similar solvent. Although many people say you cannot clean a cat to restore lost performance, the truth is that if it is marginal but still somewhat effective, pulling and cleaning it could be sufficient to get the car through emissions testing without having to buy another one.

There is a well known YouTube personality/ mechanic that says he’s seen them come back to life after removing them and then soaking in soapy detergent water overnight.

I had the dreaded weak Cat code about three years ago on my 2000 Tacoma. I went on YouTube and searched the $7 ( or whatever price) capacitor/ diode trick. No check engine light ever since.

So you’re the one polluting the air that we breathe.

And then you admit to defeating a federally required device on an open forum.