How To Find Out If The Catalytic Converter Is Bad (1995-2005 2.0L SOHC Dodge/Plymouth Neon)

Eventually, the catalytic converter on your Neon will have issues. And when it does, your Neon's OBD II fuel injection computer will turn on the check engine light and register a P0420 code.

If you're currently encountering this issue in your Neon, this tutorial will help you diagnose it. You'll be able to determine if the catalytic converter is bad or functioning correctly.

You can find this tutorial in Spanish here: Cómo Saber Si El Convertidor Catalítico Está Averiado (1995-2005 2.0L SOHC Dodge/Plymouth Neon) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.0L Dodge Neon: 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.
• 2.0L Plymouth Neon: 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

Trouble Code P0420 Basics

It's important to know that the P0420 code refers to the performance of the catalytic converter in your Dodge/Plymouth Neon. In simple terms, this code is telling you that the converter isn't adequately filtering the hydrocarbon from the exhaust gas exiting the tailpipe.

If you're wondering what 'hydrocarbons' are, it's the more 'technical' name for raw gasoline and it's the catalytic converter's main role to filter them out. I won't get into the nitty-gritty, but here's a quick look at how the converter functions:

1. Even when operating correctly, the engine doesn't fully burn all the gasoline it consumes. Although not ideal, this is normal.
2. The unburned hydrocarbons from the four cylinders move into the exhaust system and are routed to the catalytic converter.
3. Once these hydrocarbons enter the catalytic converter, they're oxidized and broken down into their basic components. These basic components are carbon dioxide and water.
4. In addition to converting hydrocarbons to something less harmful to human health, the catalytic converter also:
   • Converts nitrogen oxides (2NO_x) to nitrogen and water.
   • Converts carbon monoxide to carbon dioxide.
5. Generally, if a problem occurs in a cylinder or if a fuel injection system input sensor fails, the computer is programmed to inject more gasoline than necessary. This excessive fuel injection will lead to the catalytic converter overheating, which over time will damage it. All of this culminates in the appearance of the P0420 code.

I'm sure you've noticed water dripping from your vehicle's exhaust pipe when the engine is running. Well, this water is the result of the oxidation process that the catalytic converter is causing.

Symptoms Of A Bad Catalytic Converter

The symptoms (or lack thereof) of a bad catalytic converter depend on how the failure manifests itself.

To go into more detail: The catalytic converter usually fails in one of two distinct ways:

1. The internal filtering component of the catalytic converter wears out and stops oxidizing (filtering) the exhaust gases. In this situation, the filtering component remains intact (in one piece) and generally does not cause a problem with engine performance or power.
2. The filtering component inside the catalytic converter fractures into multiple fragments. In these cases, engine performance is affected.

Depending on how the catalyst fails, you'll notice one or more of the following:

1. The catalyst's filtering element has NOT broken into pieces:
   • The Check Engine Light is on.
   • The P0420 code is present.
   • A 'rotten egg' smell comes out of the exhaust pipe when the engine is running.
   • Fails the state mandated emissions check.
   • Bad gas mileage.
   • The engine appears to be running well. In other words, it's not lacking power, the idle is stable, and it doesn't hesitate when accelerating the vehicle on the highway.
2. The catalyst's filtering element HAS broken into pieces:
   • Lack of power. This lack of power is caused by the broken pieces of the filtering element restricting the exhaust gas.
   • The vehicle hesitates when accelerating on the highway.
   • Rough idle.
   • A rattling noise coming from under the vehicle when the engine is running or when accelerating.
   • The Check Engine Light is on.
   • The P0420 code is stored in the memory of the fuel injection computer.

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

What Causes The Catalytic Converter To Fail?

The catalytic converter will eventually fail on its own, but several things can speed up its demise:

1. An excessively rich air and fuel mixture that causes the catalytic converter to overheat. This rich air/fuel mixture is generally the result of:
   • A cylinder misfire caused by a lack of spark.
   • A faulty fuel injector that's leaking gasoline.
   • A faulty fuel pressure regulator letting gasoline pass through its vacuum inlet.
   • An EVAP system issue allowing liquid fuel (instead of vapor) into the intake manifold.
2. Engine antifreeze/coolant burning inside the cylinders and entering the exhaust system. This antifreeze now coats the reactive surface of the catalytic converter's filtering element (reducing its performance). This problem is usually caused by:
   • A blown head gasket.
   • Engine antifreeze/coolant entering the cylinders through a faulty intake manifold gasket.
3. Engine oil burning inside the cylinders and coating the reactive surface of the catalytic converter's filtering element (reducing its performance).
   • Worn piston rings that are letting oil pass and burn in the cylinder.
   • Damaged cylinder head valve seals that are letting oil leak into the cylinders.
   • Adding oil additives that contain the metal zinc.

You might be wondering what a 'rich air/fuel mixture' means. Well, this refers to a situation where the air and fuel combination that the engine needs to run contains more gasoline than normal. This excess fuel causes raw gasoline (hydrocarbons) to flow into the exhaust system and reach the catalytic converter.

Once inside the catalytic converter, this excess fuel causes the converter to overheat, leading to its eventual malfunction.

If your Neon is experiencing any of the mentioned problems, you'll need to resolve them before replacing the catalytic converter (if it's faulty). Ignoring them will result in damage to the new catalytic converter in just a few weeks.

Alright, let's move on to the next page and get started with the tests!