What is Engine Code P0420? How to Fix?

Published: 09/14/22 •  8 min read

The P0420 code is saved in the memory whenever the ECU detects lower efficiency in the catalyst system of bank 1 (the bank of cylinders where the number 1 cylinder is located). The ECU determines the effectiveness in the catalytic converter (CAT) with two sensor types: 

  1. An oxygen upstream sensor, situated prior to the catalytic converter. 
  2. Downstream oxygen sensors, placed just after the catalytic converter.

To calculate the catalyst efficiency, the ECU analyzes the results of the two oxygen sensors in real-time. If the values for both sensors are the same, the ECU determines that this catalyst’s system is inefficient.

P0420: Catalyst system efficiency lower than the threshold (Bank 1.)

P0420 Causes

The most frequent causes of P0420 trouble code are:

P0420 Symptoms

The most frequent symptoms of the data trouble code P0420 is:

Diagnose P0420

The diagnostic procedures below assume the P0420 data error code in your system. If you also have other codes related to your ignition system, emissions system, or fuel system, you must start by diagnosing them.

Based on the catalytic converter you have installed in your car, you might require at least one of these tools:

  1. OBD2 Code Reader
  2. Professional Automotive Scan Tool
  3. A Professional Infrared Thermometer Digital

Always consult the relevant OEM manual whenever you can. Diagnostic procedures from the original manufacturer must always be used first over any general workflow.

Now let’s begin the process of diagnosing the P0420 error code.

Initial steps

To get rid of a possible intermittent DTC issue, you will need to erase all ECU memory and complete an entire driving cycle.

If your Check Engine Light stays off, you may have an intermittent issue. If the light is illuminated during your drive, continue the diagnostic process.

Also read: Which Cars Are Least Likely to Have Catalytic Converter Stolen?

Visual Inspection of the Catalytic Converter 

Conduct a careful visual examination of the catalytic converter and its outlet and inlet pipe connections. Any indication of leaks in the exhaust, excessive heat, or excessive corrosion could indicate the need to replace the converter.

Don’t miss to inspect the downstream and upstream O2 sensor wires and connectors. Examine for burned, damaged, or corroded wires. Unplug the sensor and check for bent pins, loose connections, corrosion, or any other indication of a poor connection.

Scan Tool Tests

Test for Catalytic Converter 

A professional scan tool could help you save a lot of time for a CAT diagnosis. Start the engine and allow it to get back to its average temperature. Pay attention to the dow

nstream and upstream oxygen sensor values for bank 1.

The downstream O2 sensor must be continuously shifting between a lean and rich condition; however downstream O2 should be more stable, with the tendency towards mixing a less lean mixture. If both readings are too close, you could conclude that the CAT is not working correctly.

Oxygen Sensor Test for Upstream

Begin the engine and go into KOER mode. If your scanner tool can graph data, activate it. Then graph the engine’s RPM and the O2 sensor output simultaneously. Accelerate the engine until 2000 RPM. The voltage should fluctuate during acceleration, and after that, it should stabilize. The speed should decrease until it is idle. There should be reverse variations in O2 output. Repeat the test multiple times.

Certain manufacturers offer tests designed explicitly for O2 sensors. Follow the on-screen instructions to finish the tests. Note that the O2 sensor output will be completely different between narrow and wideband models. Consult the relevant OEM literature to understand the expected performance of these sensors.

Oxygen Sensor Test for Downstream

Start the engine to go into KOER mode. If your scanner tool has the ability to graph, activate it. You can graph the engine’s RPM and output of O2 at the same time. Accelerate the engine until 2000 RPM. The voltage should be relatively steady. Allow the machine to be idle again.

There should not be an increase or decrease in O2 output. Repeat the test many times. If your downstream O2 sensor output is similar to the oxygen sensor output from upstream, it could be a defective catalytic converter. Certain manufacturers offer specific tests for O2 sensors. Follow the on-screen instructions for completing these tests.

How to Fix P0420?

Based on the results of your diagnostics, you might need to do these things:

  1. Repairing oxygen sensor(s), the connectors, and/or wiring.
  2. Replacing the catalytic converter.
  3. Repairing misfiring problems
  4. Replacing the downstream and upstream oxygen sensor.
  5. Replacing muffler or leaks in the muffler.
  6. Replacing or repairing oxygen sensor connector(s)
  7. Replacing exhaust pipe or repairing its’ leaks
  8. Replacing exhaust manifold or repairing its’ leaks. 
  9. Replacing engine coolant temperature sensor
  10. Replacing or repairing the damaged wiring of the oxygen sensor(s)
  11. Replacing or repairing  leaking fuel injector(s)
  12. Repair trouble codes stored by the Power Control Module (PCM)

In the majority of late-model cars in the market, the ECU performs a fantastic job in diagnosing your catalyst’s system. In other words, when you don’t have an oxygen sensor code, it’s more likely that the catalytic converter in your vehicle is not working exactly as it ought to. We assume that this is the case, and your catalytic converter needs replacement. 

The catalytic converter is situated within the exhaust pipe following the oxygen sensor upstream It is shaped like the muffler you would find in a regular muffler.

The catalytic converter will typically be situated near the engine at the end of the exhaust.

Step 1: Make sure your ignition switch has been switched off, after which disconnect the car battery. Ensure the negative terminal is disconnected.

Step 2: Take out the downstream and upstream O2 sensor connectors. This is not required, but based on the model of your car, it might be beneficial to ensure that the sensor wiring is safe from accidental burns.

Step 3: Get rid of the old catalytic converter. Catalytic converters can be connected onto the exhaust pipe with two bolts on either side, while some are welded. Based on the type of converter you might need, cut the pipes or simply take off the bolts.

Step 4: Remove the old catalytic converter and replace it with the latest one. Make sure you have got the correct part number. Use an appropriate torque wrench for reinstalling bolts, particularly when connecting sensors, as they are very susceptible to breaking in the event of being tightened too tightly.

How Much Does it Cost to fix P0420?

P0420 could be caused by anything from a malfunctioning sensor to a failing catalytic converter. Therefore it is difficult to provide an accurate diagnosis without diagnosing the problem.

If you bring your vehicle to a repair shop to be diagnosed, the majority of shops start with a one-time frame. Based on the shop’s labor rates, it will typically cost between $75 and $150. Not all shops will apply the diagnosis fee to any needed repairs if you let them take care of your repairs. The shop can provide you with an accurate estimate of repairs to fix P0420.

After being identified, P0420 might require one or more of the following repairs to fix the root cause. These costs are based on the national averages and include components and labor. Prices may vary depending on your location and the type of vehicle.


Troubles related to the ignition system, air intake, fuel system, and misfires may cause damage to the catalytic converter when they aren’t addressed promptly. These components are the most commonly cited causes for P0420 trouble codes. When replacing a catalytic converter, replace it using the OEM model or by a replacement high-quality oxygen sensor.

It is not uncommon for the aftermarket oxygen sensors to fail. When they fail, the P0420 error code could come back. It is an excellent idea to call the manufacturer to determine the extent of the manufacturer’s warranty for parts that deal with emissions.