How To Test A P0420 Diagnostic Trouble Code (Honda 2.2L, 2.3L)

OBD II trouble code P0420 Catalyst Efficiency Below Threshold (Bank 1) is the code that let's you know that the catalytic converter on your 2.2L/2.3L Honda is not cleaning the exhaust of pollution like it should.

Usually, when a P0420 lights up the check engine, it indicates that the catalytic converter has failed, but not always since the rear (downstream) oxygen sensor could be bad (this is the O2 sensor after the catalytic converter).

In this tutorial, I'm gonna' show you some of the basics you need to know about what DTC P0420 means and several step-by-step tests to get to the bottom of this trouble code.

P0420 Basics You Need To Know

In a perfectly running engine, all of the fuel that's injected into the cylinders doesn't get completely burned. It's not the ideal thing to happen, but it's normal.

This is where the catalytic coverter saves the day since its job is to clean as much of the unburned fuel (called hydrocarbons in tech speak), that doesn't get burned in the cylinder during the combustion process.

Since a catalytic converter doesn't last forever (and certain engine conditions accelerate its death), the PCM, on your Honda 2.2L/2.3L, will set a P0420 DTC if there's an issue with its performance.

What issue sets a P0420 DTC? Specifically, that the catalytic converter is not cleaning up the exhaust of hydrocarbons like it should.

Although I'm not going into minute technical detail, on how the cat works, here are some more specifics about what it does:

  1. In a normally running engine, gasoline doesn't get burned completely when it's ignited by the spark plug's spark (inside the cylinder) and although this is not the ideal thing to happen, it's completely normal.
  2. These hydrocarbons (unburned and partially burned fuel) pass into the exhaust where they are funneled into the catalytic converter.
  3. Once in the catalytic converter, these hydrocarbons are oxidized into their basic components... which are carbon dioxide and water.
  4. In addition to converting hydrocarbons into something less harmful, the catalytic converter also oxidizes:
    1. Nitrogen oxides (2NOx) to nitrogen and oxygen.
    2. Carbon monoxide to carbon dioxide.
  5. An ignition system misfire or any other fuel control system malfunction, that causes the fuel injection computer to inject excess fuel (due to a loss of fuel trim control), will cause the catalytic converter to overheat and eventually malfunction and set a P0420 DTC.

If you've ever noticed water dripping out of the exhaust of a running car... then you're seeing the end result of the oxidation process happening inside the catalytic converter.

Symptoms Of A P0420 Diagnostic Trouble Code

The catalytic converter fails in one of two ways. The most common type of failure, on 2.2L/2.3L Hondas, is the catalytic converter simply wearing out. Which means it no longer scrubs the exhaust of those unburned hydrocarbons. The filter element inside remains intact (in one piece).

Although very rare in a Honda... the other type of catalytic converter failure is when the honeycomb filter element, inside it, shatters into several pieces.

Depending on how the catalytic converter fails... you'll see the following:

  1. The Catalytic Converter Filter Element HAS NOT Broken Into Pieces:
    1. Check engine light is shining nice and bright on the instrument cluster.
    2. DTC P0420 is present.
    3. Misfire trouble codes may be present.
    4. ‘Rotten egg’ smell coming from the exhaust.
    5. Won't pass the state mandated emmissions test.
    6. Gas mileage will suffer.
    7. There are no symptoms like: lack of power, rough idle, or hesitation.
    8. The downstream oxygen sensor shows a continuous Rich condition.
  2. The Catalytic Converter Filter Element has Broken Into Pieces:
    1. Lack of power (caused by broken pieces blocking/restricting the exhaust flow).
    2. Hesitation when accelerating the vehicle (caused by broken pieces blocking/restricting the exhaust flow).
    3. Rough idle (caused by broken pieces blocking/restricting the exhaust flow).
    4. Rattle coming from underneath vehicle when accelerating.
    5. Check engine light (CEL) is on.
    6. DTC P0420 is present.
    7. The downstream oxygen sensor shows a continuous Rich condition.

Let's take a look at the next subheading and find out why the catalytic converter fails.

What Causes the Catalytic Converter to Fail

catalytic converters don't last forever. Sorry, sooner or later, it's gonna' stop cleaning the exhaust but what accelerates its demise are several specific things:

  1. An excessively Rich condition. This usually caused by one of the following:
    1. Ignition system misfire.
    2. Leaking fuel injector(s).
    3. Leaking fuel pressure regulator.
    4. EVAP canister leaking fuel into the intake manifold.
  2. Antifreeze entering the exhaust. This is usually caused by one of the following:
    1. Blown head gasket.
  3. Engine oil.
    1. Engine oil, burning inside the cylinder from worn out piston rings, coating the reactive surfaces of the cat, reducing its performance.
    2. Engine oil, burning inside the cylinder from leaking valve stem seals, coating the reactive surfaces of the cat, reducing its performance.
    3. Adding engine oil anti-wear additives that contain zinc.

I'm gonna' explain what a Rich condition is in more detail since this is the number 1 catalytic converter killer...


In layman's terms, a Rich condition is a condition in which an excessive amount of fuel is being injected into the engine. This excess fuel, entering the engine, causes a high content of raw fuel (hydrocarbons) to pass into the exhaust and into the catalytic converter. Once in the catalytic converter, this excessive amount of fuel causes the catalytic converter to overheat and fail.

Knowing that your vehicle has any one of the above conditions/problems, will help you to make an informed decision as to what needs to get repaired to successfully solve the P0420 Catalyst Efficiency Below Threshold (Bank 1) trouble code.

Alright, let's turn the page and get testing!.....