You can find information about the P0420 fault code on this page.
It is one of the most popular fault codes that people search for on this website (in the top 50).
We have given it a seriousness of 7/10 with 1 being the least serious and 10 being the worst.
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A P0420 fault code displayed on an OBD2 reader would suggest that the car that it is plugged into is experiencing a problem with its catalytic converter.
More specifically it has detected that the oxygen level at the sensor after the catalytic converter is too high.
Modern cars have oxygen sensors at various points along the route that air (and then the fuel/air mixture) travels through the engine and exhaust to detect problems.
The catalytic convert (when working properly) is an integral part of the exhaust that converts unburnt fuel and carbon monoxide into less dangerous carbon dioxide and water vapor.
If the catalytic converter isn’t working properly (because of damage or excessive wear) the oxygen levels will appear to be higher than usual.
The P0420 error code may also be presented due to the sensor itself being faulty, its connections being damaged or the wiring connecting it to the ECU has short-circuited.
Other Signs of This Problem
Aside from the check engine light illuminating (which is what most probably alerted you to this P0420 issue), you may notice a loss of power.
As the engine is unable to work properly it may enter limp mode (depending on the make and model of the car) and will only accelerate up to 30mph~.
Standing at the rear of the car you may also notice a sulfur smell (which smells like rotting eggs) which would suggest that the catalytic converter isn’t doing its job properly and needs replacing.
What Could Be The Problem?
The problem is most probably related to the catalytic converter, the exhaust system or the sensor that detects oxygen levels after the catalytic converter.
Check for any obvious signs or damage to the exhaust – including splits or piping that isn’t connected properly together.
Further up the stream from the exhaust towards the engine, you may also find that the problem starts here as well.
A misfiring engine could be responsible for the P0420 error code if it is sending exhaust gases with too much oxygen but you should see an alternative error code at the same time.
An oil leak into the exhaust system is another possible cause of the P0420 fault code but is much less likely to happen.
Oil would prevent the catalytic converter from doing its job properly as it clogs the porous material inside of it (usually platinum and/or palladium).
(Read on for a written explanation of how to fix the P0420 fault code or check out this YouTube video for 9 solutions that you can try)
An easy fix as mentioned already is to check for splits in the exhaust system, it should be sealed tight and any holes can be easily spotted leaking smoke.
A quick sniff test to the rear of the car where the smoke is coming out of the exhaust (use caution when breathing near fumes!) can diagnose if the catalytic converter is working.
Smelling sulfur in the smoke is a strong sign that it isn’t doing its job properly and could mean you are in for a hefty bill to get the catalytic converter replaced.
Other than that the easiest checks you could do yourself include checking for obvious signs that the connections to the sensor are broken before seeking help from a trained mechanic.
We rate the P0420 fault code as being 7/10 in terms of seriousness.
You can continue to drive with this error code being present but will most likely be in limp mode to avoid causing more damage to the catalytic converter and exhaust.
It is recommended that you drive the car or get it transported to a trained mechanic who can diagnose the issue further.
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