P0420 HONDA – Causes and How To Fix The Error Code

Clyde Goering Image
Written by Clyde Goering

May 6, 2020

P0420 honda

Honda Code P0420 Definition

p0420 honda

P0420 Honda is generated in the engine regulator module when there is an issue with the effectiveness of the catalytic converter. The locomotive regulator unit utilizes two 02 sensors, one at the back of the catalytic converter and the other one in its front part- to measure the effectiveness of the catalytic converter. In case one sensor is defective and getting inaccurate values, or the catalytic converter is faulty, an error code is generated.

  • The O2 sensor at the back of the catalytic converter is an analytical 02 sensor for this error code.
  • The obverse O2 sensor helps to measure the mixture, and it tells the locomotive regulator unit if the locomotive is running with a lean or rich mixture and adjusts it.

What Does P0420 Mean For Your Honda?

When the rear sensor is getting inaccurate values, the P0420 error code will be triggered in the engine regulator unit. It could be as a result of the faultiness of the catalytic converter, or a wrong alarm since one of the sensors is spoiled. Also, there could be several issues with your engine that are damaging the catalytic converter, and if you decide to install another catalytic converter, it may also be destroyed.

Many people believe that rich or lean mixtures could generate the P0420 Honda code. Yes, they can trigger the trouble code but in a different way than you think. Lean or rich mixtures can destroy the catalytic converter, and when it is filled with fuel or damaged, the effectiveness will be low, and the back O2 sensor will notice that and display the P0420 Honda code.

Symptoms of P0420 HONDA

You will possibly get no symptoms except the “check locomotive light” with the error code itself, and you might have other issues with your locomotive that is destroying the catalytic converter that can have symptoms such as;

  • Check locomotive light
  • Rich mixture
  • Misfires
  • Slow acceleration
  • Engine oil burn/ a bluish smoke from the exhaust pipe
  • Lean mixture

Always fix the above issues first.

Possible causes of P0420 HONDA

The possible cause of P0420 Honda is the catalytic converter. It could be an old obsolete catalytic converter, but still, I have noticed many issues with a catalytic converter, but it is not an original honest part. Cheaper catalytic converters may be inadequate, and in some instances, you have to purchase a tremendous catalytic converter from the producer. There are catalytic converters that have been fixed a bit far at the rear on the exhaust pipe. Due to this, the catalytic converter will not get heated enough and will generate a P0420 Honda trouble code.

  • Destroyed catalytic converter (most possible)
  • Exhaust leak
  • Dishonest catalytic converter
  • Intake leak
  • Destroyed downstream back O2 sensor/ defective wings
  • A defective fixed placing of the catalytic converter
  • Destroyed upstream obverse O2 sensor/defective wings

Possible solutions P0420 Honda code

Experts recommend that you should not replace the faulty parts without conducting a proper investigation; it may waste your cash if you are unlucky. However, I have listed possible solutions to the problems. They include;

  • Fix misfires
  • Fix oil burn
  • Replace the back O2 sensor
  • Substitute catalytic converter
  • Repair defective wirings
  • Substitute to a genuine catalytic converter
  • Replace obverse O2 sensor
  • Replace locomotive control unit- this is rare
  • Check the information with an OBD2 scanner from amazon

Common diagnosis mistakes of P0420 HONDA

Honda codes include; P0171, P0174, P0308, P0307, P0306, P0305, P0303, P0302, P0301, and P0300. These P0420 Honda codes can provide inaccurate data readings that trigger conditions which generate error code P0420. If you find other codes, you should address them first.

How to fix error code P0420

While fixing this error code, you might require some important tools to ease the diagnosis, and it will assist you in conducting an excellent diagnosis test.

OBD2 Scanner
  • You may need an OBD2 scanner, which is a vital instrument to examine the error code and to view live information and other parameters. I would recommend acquiring an investigative tool that can display the signals in the form of graphs to ease the diagnostics exercise.
  • A digital multi-meter is a necessary tool for any electrical measurements, and it is categorically a tool to capitalize on. You require a digital multi-meter for nearly all troubleshooting in your vehicle electrical, and they are even not that costly. Purchase a multimeter depending on your necessities; there are both expensive and cheap multi-meters.
  • You always need to have a vehicle battery charger when performing any workouts of your car. Low battery power may cause other error codes that will confuse you and can lead you to defective diagnosis. The low battery voltage can also destroy engine control units or other car electronics if you are unlucky.
  • You can try to utilize a catalytic converter cleaner any time you think your catalytic converter is dirty or has oil from previous internal oil leakages. The cleaner can also be used for other jobs, including cleaning your exhaust releases.
  • Another essential tool for fixing the P0420 Honda error code is a digital laser thermometer. A digital laser thermometer helps to diagnose the temperature of your catalytic converter. Although some of the best OBD2 scanner gives you a temperature value, it is only the calculated value, and it is not the actual value. The laser thermometer can also be useful for other tasks, such as when troubleshooting your vehicle. It is perfect for troubleshooting your coolant system.
  • Lastly, you may need an O2 sensor adaptor that can be used to track the locomotive regulator unit, and it might fix your Honda trouble code. However, an O2 sensor adaptor is not the recommended tool since it may break some rules depending on which nation you reside in.


In many cases, the catalytic converter is defective and the source of the P0420 Honda error code. Still, there can be another cause of why the catalytic converter is getting spoiled. Ensure that you always fix all other possible error codes before you fix the P0420 Honda error code. You can utilize special equipment or reprogram the locomotive regulator unit to trick the regulator unit into solving the P0420 Honda trouble code. Bank 2, Bank 1 is directing to which of the two O2 sensor adaptors that are defective or which catalytic converter that is defective. Additionally, the catalytic converter cleans the exhaust releases and may not be eliminated by the code.

Clyde Goering Image
Written by Clyde Goering


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