P0420 -What Does It Mean? (1996-1997 2.7L Honda Accord)

When the catalytic converter fails on your 2.7L V6 Honda Accord, the fuel injection computer will set a trouble code P0420: Catalyst System Efficiency Below Threshold.

In this article, I'll explain what causes the catalytic converter to fail and what you should do before replacing it (to avoid a repeat of the problem).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.7L V6 Honda Accord: 1995, 1996, 1997.

What Does The Catalytic Converter Do?

As you're probably already aware, the 2.7L engine in your Honda Accord creates and emits harmful pollutants as it runs.

These pollutants are: hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).

Hydrocarbons (raw gasoline) are the result of the fuel that doesn't get completely burned in the combustion process.

The catalytic converter is tasked with reducing these harmful pollutants and converting them into harmless gases.

Code P0420 indicates that the catalytic converter is not functioning efficiently and thus not reducing the harmful pollutants your Honda's engine is emitting.

Common Symptoms Of A P0420 Trouble Code

You're gonna' see one or more of the following symptoms:

• The check engine light is on.
• If catalytic converter has broken into pieces, you'll notice a lack of power when you accelerate the engine under load.
• Bad gas mileage.
• Rotten egg or sulfur smell coming out of the tailpipe when the engine is running (especially when you accelerate the engine under load).

What Causes A P0420 Trouble Code?

The only cause of a P0420 trouble code is a failing or failed catalytic converter.

A bad oxygen sensor will not cause a P0420 trouble code (although it could be contributing to the catalytic converter's demise).

In my experience, the number one cause of a bad catalytic converter is too much raw fuel (hydrocarbons) entering the exhaust and overloading/overheating the catalytic converter.

In troubleshooting a P0420 trouble code, it's important that you check if one of the following issues is present:

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

Before You Replace The Catalytic Converter

Before you replace the catalytic converter, it's important that you first address what made it fail.

If you don't, then the new catalytic converter will also fail and the check engine light will illuminate with a P0420 trouble code.

I recommend that you troubleshoot and resolve any problem that could be:

• Overloading the exhaust with hydrocarbons (raw gasoline).
• Causing anti-freeze to enter the exhaust gas.
• Causing engine oil to enter and burn in the engine cylinders.

In my experience, the most common cause of a bad catalytic converter has been a misfire problem. So if you have a misfire trouble code stored in the fuel injection computer's memory (P0300, P0301, P0302, P0303, P0304, P0305, P0306), solve them first.

If any other trouble codes are present along with the P0420 trouble code, troubleshoot and resolve them first before replacing the catalytic converter.

More 2.7L V6 Honda Accord Tutorials

You can find a complete list of 2.7L V6 Honda Accord tutorials in this index:

• Honda 2.7L Index of Articles.

Here's a small sample of the tutorials you'll find in the index:

• Ignition System Wiring Diagram (1995-1997 2.7L Honda Accord).
• How To Test Engine Compression (1995-1997 2.7L Honda Accord).
• How To Test The Head Gaskets (1995-1997 2.7L Honda Accord).
• How To Test The Ignition System (1995-1997 2.7L Honda Accord).