Check engine code P0420: how to fix, most common reason
Check engine code P0420: how to fix, most common reason

Check engine code P0420: how to fix, most common reason

The exhaust system of a modern car is a complex set of devices serving not only to divert gases and reduce noise levels, but also to control the correct operation of the engine. The task of the catalytic converter is to prevent harmful substances from entering the atmosphere. If the code P0420, which deciphers as “the efficiency of the catalytic converter system is below the threshold level” occurs, it indicates a poor capacity of the catalytic converter.

Where does code P0420 come from?

Some vehicles have two oxygen sensors. In this case, code P0430 will tell about the malfunction of the second oxygen sensor. Most often the appearance of this code indicates the failure of the filter element or its insufficient efficiency, but in some cases it can be associated with the use of low-quality fuel.

Oscillogram of the oxygen sensor voltage check

The catalytic converter is designed to last between 200,000 and 250,000 km, but only under normal operating conditions. If the car is often filled with gasoline with a high lead level, the catalytic converter will run out much faster. In addition, low-quality fuel contributes to reduced compression and ignition timing, which subsequently affects the catalytic converter.

Possible reasons for this:

  • Use of leaded fuel.
  • Damage or failure of the S2 oxygen sensor.
  • Damage to the catalytic converter.
  • Disruptions in the operation of the exhaust system components.
  • Prolonged operation of the machine with a misfire in the cylinder.

sensor focus 3
Schematic of the catalytic converter with two oxygen sensors

Consequently, the code can occur in many cases. Most often it is failure or wear of the neutralizer itself, as well as the use of low-octane low-quality gasoline. 

If it is really a matter of poor quality gasoline, all you have to do is fill up with good fuel and the code will reset.

ошибка 0420
Oxygen sensor

Signs of failure P0420

When the catalytic converter is clogged or begins to deteriorate, the driver may notice the following signs:

  • Noticeable increase in gasoline consumption.
  • Decrease the engine dynamics.
  • The very unpleasant smell of car exhaust.
  • When the catalyst begins to crumble, you can feel a rattle from its side.
  • Problems with idling may occur.
  • Difficulties with starting the power unit are probable.
clogged catalytic converter
Clogged catalytic converter

How to check the catalytic converter

Before making a final diagnosis, it is necessary to know how to check the catalytic converter. To assess if it is working, you need to compare the output voltage between the upper and lower O2 sensor. It is also necessary to look at the fuel supply correction data.

Oxygen sensor structure
Oxygen sensor structure

The voltage at the output of the sensors is read by the onboard computer and varies depending on how enriched the fuel-air mixture is. Under normal conditions, if the mixture is enriched, the voltage is about 900 mV. A voltage of 100 mV, indicates that the mixture is lean. Fluctuations within this range are quite normal.

The fuel correction ideally tends to “0”, a deviation of 10% is acceptable if the power unit has a solid mileage. If the correction exceeds 25%, it is abnormal, and indicates that there are problems in the mixture formation. At the same time it is worth paying attention to the presence of other errors.

ошибка 0420
Failed oxygen sensor and plug

Removing failures P0420 and P0430

In order to get the code removed, some drivers unknowingly start doing things that are not necessary at all. However, the first thing to pay attention to is the oxygen sensor. Since the upper and lower sensors are interchangeable, they should be swapped. If they are the problem, the code will change.

Plugs instead of the oxygen sensor

It is also worth checking the capacity of the catalytic converter. Problems are mainly caused by abnormal functioning of the system in front of the catalytic converter. Excessive engine temperatures can cause P0430 errors and cause the catalytic converter to fail. Most often the excessive temperature is due to ignition skips in the cylinders.

If the code P0420 has appeared on VW, Ford, Subaru, and any modern car, the original catalytic converter can be very expensive. At the same time, it is possible to choose a universal model, or install a second lambda bump or a flame catalytic converter. Also ECU reprogramming for another standard of toxicity can be a solution.

One comment

  1. Adam M


    ….Renault Scenic 2 ; silnik 2.0 16V ; ph 2 ,
    wyświetlił się P420 , szukałem przyczyny i znalazłem
    BENZYNA ZŁEJ JAKOŚCI , DO TANKOWAŁEM 30l dobrego pamiwa i po 10 km bład sam zniknoł .

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