Error P0420: what to do about it?

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Error P0420: what to do about it?
Error P0420: what to do about it?

The car exhaust gas cleaning system is relatively complex, and the catalyst is one of its elements. It is also called a catalytic converter. Its main task is to turn harmful substances emitted into the atmosphere into less harmful ones, and error P0420 indicates that this exhaust gas cleaning element is working poorly or not working at all. In some cars, two converters are used at once. In this case, the error code may be P430. If such an error occurs, then first of all it indicates that the life of the catalyst has come to an end. Of course, there is a chance that the error arose due to the poor quality of gasoline (and this happens), but most often the problem is precisely in the "death" of the catalyst. Well, or in low efficiency.

error p0420

Why is error P0420 generated?

The "brains" of the car in the process of operation (control) compare the signals from two sensors for a certain period of time, calculate the duration of the voltage signals, and if it is notfits into the specified threshold, the system considers this a violation of the neutralizer. It is believed that the difference between the amplitudes of the oxygen sensors (front and rear) should not be more than 0.7 times per minute. However, the Check engine light does not come on immediately, but within 100 seconds. In this case, the load on the engine should be from 21 to 63% at a crankshaft rotation speed of 1720-2800 rpm. Also, the temperature of the catalyst must be over 500 degrees.

If the catalytic converter is worn out, the reading of the rear oxygen sensor will gradually approach the reading of the front one. The main purpose of the catalyst is to oxidize carbon monoxide and neutralize CO2 emissions into the environment. Starting from the Euro-3 standard, this process is monitored by two sensors, between which the signals are constantly compared in order to register the convergence of readings between them. Therefore, sooner or later, in any case, error P0420 will occur: on Ford Focus 2, Nissan, Chevrolet, Honda, Toyota and other cars that were produced after 1996 and have two lambda probes (2 sensors).

p0420 error

So, the main cause of error P0420 is the detection of unburned fuel and oxygen residues in the exhaust gases. And yes, it's only a matter of time, because the catalyst has a limited service life. And this service life largely depends on the quality of gasoline that the owner of the car refuels.

Signs of error P0420. How does the car behave?

Depending on how exactly the catalyst "dies" (clogged orits destruction begins), the car can behave differently. But the first signal is the Check engine light on the instrument panel. Some cars even have a special catalyst overheating light, so you don’t even have to diagnose an error. This means that the exhaust gases are no longer Euro 3-5 compliant.

p0420 error ford

Basically, when the error code P0420 appears, in parallel with it:

  1. Higher fuel consumption. If the car usually consumes 8 liters per 100 km, then with an inoperative catalyst, consumption can increase to 9-10 liters per 100 km.
  2. Vehicle dynamics are reduced.
  3. Exhaust smell changes and becomes more pronounced.
  4. Catalyst side rattling.
  5. You may experience erratic idling (rpm jumps).

If at least a few of the above signs are observed, then this indicates a problem with the catalyst. Therefore, vehicle diagnostics are required.

Causes of engine error P0420

During normal operation of the engine, the catalyst has a service life of 200-250 thousand kilometers. However, when refueling with a high lead content, the valve will die much faster. Also, due to possible malfunctions in the ignition and distribution of gases, compression may be impaired. As a result, there will be misfires, which also accelerates the destruction of the catalytic converter and entails error P0420 on the Ford Focus 2 andother vehicles.

error code p0420

Therefore, low-quality gasoline, which motorists systematically fill their cars with, is the first cause of catalyst damage. It can break down after 80 thousand kilometers, although it is initially calculated at 200-250 thousand, provided that the car is running on normal gasoline.

Causes of error P0420:

  1. Using leaded gasoline.
  2. Oxygen sensor failure S2.
  3. Short circuit in the "lower" oxygen sensor system.
  4. Damage in the system of another element: exhaust manifold, pipe, muffler, etc.).
  5. Catalyst damage.
  6. Continuous operation of internal combustion engines with misfires.
  7. High fuel pressure.

From this it follows that there may be 7 causes of error P0420 on Ford Focus 3 and other cars where lambda probes are used in the exhaust gas cleaning system. However, most often everything is much simpler, and drivers just need to refuel with good gasoline or put an oxygen sensor snag. In rare cases, the lambda contacts just don't sit well, which is why the system does not see them and displays an error.

p0420 error ford focus

But in order to correctly and determine the problem that caused this error, you need to diagnose the car. In certain cases, it is necessary to check the exhaust system, manifold or oxygen sensor for leaks. Suctions and leaks could well affect the operation of the sensors, which, as expected, led toerror P0420. But most often it is the catalyst that is to blame.

Tips for troubleshooting

Before you run to the car service, try to find out the reason yourself. To do this, you need to carry out a few simple checks. first, try to remember at which gas station you last refueled and whether the correct fuel was filled. If earlier you always filled in A98 gasoline, and the last time you decided to test A92, then it is quite logical that the system showed error P0420. In this case, just roll the rest of the A92 and fill this time with A98. For many car owners, after changing gasoline, the error disappears.

code p0420 ford focus 2

Next, check the rear oxygen sensor connector. If he moved away a little, then this could well cause an error. If everything is fine with this, then you will have to connect a computer to remove errors and data from the control unit.

Checking the operation of the catalyst

To check the correctness and efficiency of the catalyst, compare the voltage graphs between the two oxygen sensors. The computer will clearly show a decrease in the output voltage when the mixture is lean and an increase when enriched. If the oxygen sensor voltage is in the region of 900 millivolts, then this indicates an enrichment of the mixture, 100 millivolts indicates a lean mixture.


Many car owners, out of ignorance of the cause of the error, try to eliminate it by replacing sensors or cleaning the damper. But this does not help, because the reason lies elsewhere.

First, you need totry swapping the lambda probes. They are identical in their type and can replace each other. If the second oxygen sensor was faulty, then we will get another error (as an option, P0134). Such diagnostics are simple and effective only if the second sensor has failed. If the problem is not in the sensors at all, then the error will not disappear.

engine error p0420

Secondly (it has already been said), you need to try to refuel with better quality gasoline. If the reason was in the fuel, then after 2-3 days the error will disappear.

The third step towards solving the problem (if nothing helped) is to check the catalyst. We need to check its throughput. Its destruction could be due to the malfunction of other engine systems. And if this element was overheated, then a similar error could well have occurred.

Solve the problem of low catalyst efficiency

Often the problem of a catalyst with poor efficiency is solved by flashing the ECM. Simply another software is set, where the toxicity norm is different (for example, EURO2). The system still compares the value of the two sensors, but now the difference in parameters will correspond to the toxicity of EURO2. What can be achieved with this? At a minimum, the error on the dashboard will disappear, but no more.

Catalyst replacement

The most expensive option is to replace the old catalyst with a new and original one. However, this is an expensive procedure due to the high cost of the device itself. The price can reach 40 thousand rubles.

Cheaperthe option is to use a universal catalyst, which has a lower efficiency than the original one (it's all about the material: the original catalyst is ceramic, and the universal one is made of metal). Also, its resource is only 30-50 thousand kilometers, and not all cars take it well. But it does not require any software changes in the system. And another option: during disassembly, you can find the original catalyst, however, it was in use. It will not be known what its mileage is and how soon it will fail.

Fire arrester installation

If you don't care about toxicity standards at all, then a cheap and concise option is to install a flame arrester. To do this, a bank of catalyst is cut out and a blende of the second lambda is installed. This is called hardware snag, but there is also software. We are talking about flashing the system and transferring it to a reduced toxicity rate. It is worth noting that this solution to the problem, although it involves more severe air pollution in the street, but greatly increases engine power. After all, all these toxicity standards and compliance with environmental standards cut the engine's capabilities.

Removing the catalyst from the system

Alternatively, the catalyst can be completely removed from the exhaust system and a two-channel emulator can be installed, with which you can adjust the signal speed, response time.

It may also be that during the diagnostic process it turns out that the throughput of the catalyst is normal (about 0.21 kg/cm2 at 2000 rpm). This is quite possible, becausethe error is triggered even if the catalyst is running at 70% of its capacity. In this case, you can put a special spacer under the lambda probe. This is a very cheap solution, but it is not a panacea.


Now you know what P0420 (error) means. Ford, Toyota, Honda, Chevrolet and other brands often suffer from it. However, there is nothing wrong here. The catalyst is not an element of the system to worry about. And even if there is no money to replace it with an original spare part, it can be replaced with a universal one or completely drowned out (software or hardware).

We hope all of the above troubleshooting methods for this error helped you. There are many options for solving it, so you should not panic because of such a trifle. It is quite possible that you simply roll back bad gasoline and after refueling a new one, the error will disappear. But this rarely happens. Usually the catalyst is jammed, and this service at different service stations is not expensive, because it does not imply anything complicated. Good luck in fixing the error.

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