Error code P0420: signs, causes and fixing the catalyst low efficiency

Error code P0420 is deciphered as: «catalytic converter system efficiency below threshold level». This indicates very poor catalytic converter efficiency. If the vehicle is equipped with two neutralizers at once, this error may also have the code P0430. The system displays this error as “Catalyst System Efficiency Below Threshold (Bank 1) or (Bank 2)”.

It is not difficult to guess that the problem will probably indicate the end of life of the catalyst, although there is hope that it is due to poor fuel or the failure of the second lambda.

Conditions of DTC P0420 formation

P0420 — error code «low catalytic converter performance»

ECM, while controlling, compares signals of 1st and 2nd sensors during the set time interval, calculating voltage signal duration, and if it goes out of the set threshold, the «brain» of the car, interprets it, as a violation of the neutralizer. The threshold value of the difference between the amplitudes of the front S1 (taken as a reference) and rear S2 oxygen sensors is more than 0.7 times in one minute.

However, the check lamp that signals the fault indication in the ECM memory, does not light up immediately, but only when the catalytic converter efficiency is reduced for 100 seconds and the engine load must be between 21 and 63% with a crankshaft rotation of 1,720 — 2,800 rpm and the catalytic converter temperature exceed 500 degrees Celsius.

As the catalytic converter wears out (reducing its capacity ), the rear sensor readings approach the readings of the front oxygen sensor.

Signal from the oxygen sensor

When the catalytic converter is functioning normally, the heated oxygen sensor output signal switches slowly between enriched and depleted state values. Frequent lambda probe switching between these states is an indicator of a decrease in neutralizer efficiency. As a result, its ability to accumulate oxygen is reduced.

The task of the catalyst is to oxidize carbon monoxide and neutralize CO2 hydrocarbon emissions in order to reduce the concentration of harmful substances. This process, starting from the Euro 3 standard, is monitored by two oxygen sensors. The signals of the first and second lambda are constantly compared in order to register the convergence of their readings. Therefore, error code P0420 will bother all car owners, including such as: Nissan, Toyota, Chevrolet, Ford, Honda or others produced after 1996 and having 2 lambda probes in the exhaust system.

Fault code P0420 appears when oxygen and unburned fuel residue are detected in the exhaust gas.

How the car behaves (signs of p0420 error)

Depending on whether the catalytic converter is clogged or has begun to destroy, the car will show certain signs of malfunction. Exhaust gas will no longer meet EURO 3-5 standards when the instrument panel lights up check engine or in some cars and catalyst overheating lamp. In general, there can be several signs of the error P0420 (P0430):

Location of the catalyst and oxygen sensors

  • Increased fuel consumption;
  • decreased engine dynamics;
  • change of the exhaust smell (became very unpleasant);
  • rattling on the catalytic converter side (if it crumbles);
  • sometimes unstable idling run;
  • there can be difficulties with starting the engine. So, if you observe more than two of these signs at the same time, probably the car has a problem with the catalyst, so a diagnostics is needed to find out the exact code of the problem!

The main causes of code P0420

If used properly, the life of the catalytic converter is at least 200-250 thousand km, but if you regularly refuel with fuel, which will contain impurities with a high concentration of lead, its destruction will be accelerated. Also, due to bad fuel or malfunctioning ignition and gas distribution, compression fault and ignition misses occur, which directly contributes to the reduction of the catalytic converter service life, as well as to the appearance of the error code P0420 or P0430.

Low-quality gasoline is the main danger for catalysts! If you are constantly filled up with bad fuel, even at 80 thousand kilometers the catalytic converter can already be spoiled.

P0420 error detection with a diagnostic tool

So, here are the main possible reasons:

  1. Use of leaded gasoline.
  2. Damaged or faulty oxygen sensor S2.
  3. Short circuit of «lower» oxygen sensor.
  4. Damaged catalytic converter.
  5. Damage in exhaust system (exhaust manifold, muffler, pipes, gaskets, etc.).
  6. Long-term vehicle operation with ignition skips in cylinders.
  7. Excessive fuel pressure in the system.

As you can see, there may be at least 7 reasons why the error P0420 pops up. Often the easiest thing for car owners to do is to replace the catalyst, cut it out or install a bump. And the luckiest, when this malfunction occurs at not so high mileage, it is enough to fill up with good gasoline or check the contacts of the lambda sensor.

In order to correctly determine the malfunction that caused the appearance of this code, it is necessary to perform a thorough diagnosis of all possible causes.

Common causes of P0420 by car model

There are several models of the cars, which more frequently has code P0420:

  • Toyota Corolla — has a bad catalytic converter (also because of the oil going through the piston rings). Vacuum and exhaust leaks checking is recommended.
  • Ford Focus — faulty air-fuel mixture caused by vacuum leaks. Use the diagnostic scanner to identify air-fuel mixture codes.
  • Subaru Forester — the same problems as Toyota Corolla has. Check vacuum and exhaust leaks.
  • VW (Skoda, Audi, Seat) 1.8T, 2.4 V6 — problems with oxygen sensor or an exhaust leak. Check the crankcase ventilation — it must be free from dust. Also make sure that the check valves are functioning in the proper way.

Methods of checking and solving the problem

In some cases, it is necessary to check the oxygen sensor or the exhaust system and manifold for leaks. Leaks and air inflows can affect the O2 sensors and cause a P0420 code, but more often, it's only a direct result of the catalytic converter condition.

Some tips on how to fix the problem

Before you start the diagnosis, in order to save time on finding out the cause and fixing the problem, we advise you to make some simple checks. So:

  1. First of all, remember at what gas station you filled up and whether you poured the same fuel as always. And also make sure that there are no symptoms of coolant ingress in the engine, as well as oil leakage.
  2. Check the ignition dwell angle. Retarding below the required timing can increase the temperature of the exhaust and, over time, reduce the efficiency of the catalytic converter.
  3. Check the connector of the 2nd («rear») oxygen sensor. After making sure that all this was unchanged, you will have to connect the computer to take data from the electronic control unit.

Checking the performance of catalytic converter and its parameters

Oxygen sensor readings on an oscilloscope

To assess the performance of the catalytic converter, it is necessary to compare the voltage output graphs between the «upper» and «lower» oxygen sensors, and also look at the fuel supply correction data.

The oxygen sensor output voltage will decrease when the mixture is lean and increase when it is enriched. The normal reading for the oxygen sensor will oscillate between 900 millivolts (enriched state) and 100 millivolts (lean state).

The short-term correction, ideally, should tend to «0», but on an engine with mileage deviations up to 10% is acceptable. And when the fuel correction exceeds 25%, there is also a long term correction, so if both values are present, it indicates a fuel-air mixture problem. Therefore, pay attention to additional fault codes.

How you can fix the problem associated with the P0420 error

Some car owners, unaware of the origin of the P0420 error, may start fixing the problem by cleaning the choke or replacing the sensors that affect air and fuel flow.

Instead, perform the following replacements:

First — swap out the lambda probes, upper and lower, because they are the same and replaceable. Therefore, if the problem is in the second oxygen sensor, then the error code will change (error P0134 may appear).

Second — refuel with another petrol, of higher quality, and drive for a while (if it's because of the fuel, the situation will change in a couple of days).

The third step on the way of fixing can be to check the catalyst, or rather, its capacity (there are several methods). Internal catalytic converter failure usually occurs due to abnormal engine systems work before the catalytic converter. High temperature of the neutralizer is usually the cause of its failure and the appearance of this code. For example, ignition skips can lead to an increase of working temperature of the catalytic converter.

How to solve the problem of low catalytic converter efficiency

Most often the error P0420 pops up because of the end of life of the catalyst, so the problem of low efficiency should be solved by replacing a new one or reflashing the ECM to a different standard of toxicity (under EURO2).

The most expensive (from 300$ to 2000$) method of repair will be replacement of the catalytic converter with a new original one.

The cheaper way (200$) — replacement to a universal catalytic converter. Its performance is a bit worse (the original ceramic, and these are often made of iron), the resource is only about 30-50 thousand km, and not all cars will work properly after this change. But on the other hand, you will not have to make any software changes. The used original catalyst will cost the same at a junkyard.

If the emission standards are not important to you, then a relatively inexpensive option would be to install a flame arrestor. This solution involves cutting out the catalyst can and installing a second lambda cheat.

The budget variant, including reprogramming and installation of an oxygen sensor simulator (mechanical or electronic), is to convert the car to the lowered norm of toxicity. This method provides the removal of the catalyst from the exhaust system, and the installation of a two-channel emulator will give the opportunity to configure a more specific level of response time, signal speed and its displacement.

And if the capacity of a catalytic converter is still within the norm (0,21 kg/cm² at 2000 rpm), you can put a spacer under lambda probe temporarily. This way is the cheapest, but not a magic pill and suits not for every car. In addition, you can try to fix the code P0420 as it shown in this video:

We hope that all the above methods of fixing the code P0420 will help you, and you will no longer have questions like: «why did p0420 error appear and how to fix it».Share the obtained knowledge in social networks with your friends. Write us a review about whether the problem was solved, how much it cost and what method helped you to solve the cause of the weak efficiency of the catalyst.


  • How much does it cost to fix P0420?

    Fixing the code P0420 in most cases means the replacement of the catalytic converter. At most maintenance services it costs from 300$ to 2400$.

  • Repair costs for P0420

    Repairing can include 2 replacements: Front oxygen sensor — from 150$ to 300$, Rear oxygen sensor — from 150$ to 300$.

  • How to clear error code P0420?

    You can clear code P0420 only with the help of an OBD2 scanner.

  • Can I drive with P0420?

    Yes, you can, but not for long distances. To prevent serious damages you should fix an error as soon as possible.

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